1 The LORD reigneth; let the people tremble: he sitteth between the cherubims; let the earth be moved. 2 The LORD is great in Zion; and he is high above all the people. 3 Let them praise thy great and terrible name; for it is holy. 4 The king's strength also loveth judgment; thou dost establish equity, thou executest judgment and righteousness in Jacob. 5 Exalt ye the LORD our God, and worship at his footstool; for he is holy. 6 Moses and Aaron among his priests, and Samuel among them that call upon his name; they called upon the LORD, and he answered them. 7 He spake unto them in the cloudy pillar: they kept his testimonies, and the ordinance that he gave them. 8 Thou answeredst them, O LORD our God: thou wast a God that forgavest them, though thou tookest vengeance of their inventions. 9 Exalt the LORD our God, and worship at his holy hill; for the LORD our God is holy.
[AD 430] Augustine of Hippo on Psalms 99:1
"The Lord is King, be the people angry" [Psalm 99:1]. For our Lord Jesus Christ began to reign, began to be preached, after He arose from the dead and ascended into heaven, after He had filled His disciples with the confidence of the Holy Spirit, that they should not fear death, which He had already killed in Himself. Our Lord Christ began then to be preached, that they who wished for salvation might believe in Him; and the peoples who worshipped idols were angry. They who worshipped what they had made were angry, because He by whom they were made was declared. He announced, in fact, through His disciples, Himself, who wished them to be converted unto Him by whom they were made, and to be turned away from those things which they had made themselves. They were angry with their Lord in behalf of their idols, they who even if they were angry with their slave on their idol's account, were to be condemned. For their slave was better than their idol: for God made their slave, the carpenter made their idol. They were so angry in their idol's behalf, that they feared not to be angry with their Lord. But the words, "be they angry," are a prediction, not a command; for in a prophecy it is that this is said, "The Lord is King, be the people angry." Some good results even from the enraged people: let them be angry, and in their anger let the Martyrs be crowned....You heard when Jeremiah was being read before the reading of the Apostle, if you listened; ye saw therein the times in which we now live. He said, "The gods that have not made the heavens and the earth, let them perish from the earth, and from under the heaven." [Jeremiah 10:11] He said not, The gods that have not made the heavens and earth, let them perish from the heaven and from the earth; because they never were in heaven: but what did he say? "Let them perish from the earth, and from under the heaven." As if, while the word earth was repeated, the repetition of the word heaven were wanting (because they never were in heaven): he repeats the earth twice, since it is under heaven. "Let them perish from the earth, and from under the heaven," from their temples. Consider if this be not now taking place; if in a great measure it has not already happened: for what, or how much, has remained? The idols remained rather in the hearts of the pagans, than in the niches of the temples.

[AD 430] Augustine of Hippo on Psalms 99:2
"The Lord is great in Sion, and high above all people" [Psalm 99:2]....He whom I spoke to you of as above the Cherubims, is great in Sion. Ask thou now, what is Sion? We know Sion to be the city of God. The city of Jerusalem is called Sion; and is so called according to a certain interpretation, for that Sion signifies watching, that is, sight and contemplation; for to watch is to look forward to, or gaze upon, or strain the eyes to see. Now every soul is a Sion, if it tries to see that light which is to be seen. For if it shall have gazed upon a light of its own, it is darkened; if upon His, it is enlightened. But, now that it is clear that Sion is the city of God; what is the city of God, but the Holy Church? For men who love one another, and who love their God who dwells in them, constitute a city unto God. Because a city is held together by some law; their very law is Love; and that very Love is God: for openly it is written, "God is Love." [1 John 4:8] He therefore who is full of Love, is full of God; and many, full of love, constitute a city full of God. That city of God is called Sion; the Church therefore is Sion. In it God is great....

[AD 430] Augustine of Hippo on Psalms 99:3
All these very people, over whom You are great in Sion, "Let them confess unto Your Name, which is great" [Psalm 99:3]. Your Name was little when they were enraged: it has become great; let them now confess. In what sense do we say, that the Name of Christ was little, before it was spread abroad to so great an extent? Because His report is meant by His Name. His Name was small; already it has become great. What nation is there that has not heard of the Name of Christ? Therefore let now the people confess unto Your Name, which is great, who before were enraged with Your little Name. Wherefore shall they confess? Because it is "wonderful and holy." Your very Name is wonderful and holy. He is so preached as crucified, so preached as humbled, so preached as judged, that He may come exalted, that He may come living, that He may come to judge in power. He spares at present the people who blaspheme Him, because "the long-suffering of God leads to repentance." [Romans 2:4] For He who now spares, will not always spare: nor will He, who is now being preached that He may be feared, fail to come to judge. He will come, my brethren, He will come: let us fear Him, and let us live so that we may be found on His right hand. For He will come, and will judge, so as to place some on the left hand, some on the right. [Matthew 25:31-33] And He does not act in an uncertain manner, so as to err perchance between men, so that he who should be set on the right hand, be set on the left; or that he who ought to stand on the left, by a mistake of God should stand on the right: He cannot err, so as to place the evil where He ought to set the good; nor to place the good, where He should have set the evil. If He cannot err, we err, if we fear not; but if we have feared in this life, we shall not then have what to fear for. "For the King's honour loves judgment."...

[AD 397] Ambrose of Milan on Psalms 99:5
And the apostles worshiped, and, therefore, they who bore the testimony of the faith received authority of faith. And the angels worshiped God, of whom it is written: “And let all his angels worship him.”But they worship not only his Godhead but also his footstool, as it is written: “And worship his footstool, for it is holy.” Or, if they deny that in Christ the mysteries also of his incarnation are to be worshiped, in which we observe as it were certain express traces of his Godhead and certain ways of the heavenly Word, let them read that even the apostles worshiped him when he rose again in the glory of his flesh.

[AD 430] Augustine of Hippo on Psalms 99:5
"O magnify the Lord our God" [Psalm 99:5]. Magnify Him truly, magnify Him well. Let us praise Him, let us magnify Him who has wrought the very righteousness which we have; who wrought it in us, Himself. For who but He who justified us, wrought righteousness in us? For of Christ it is said, "who justifies the ungodly." [Romans 4:5] ..."And fall down before His footstool: for He is holy." What are we to fall down before? His footstool. What is under the feet is called a footstool, in Greek ὑ ποπόδιον, in Latin Scabellum or Suppedaneum. But consider, brethren, what he commands us to fall down before. In another passage of the Scriptures it is said, "The heaven is My throne, and the earth is My footstool." [Isaiah 66:1] Does he then bid us worship the earth, since in another passage it is said, that it is God's footstool? How then shall we worship the earth, when the Scripture says openly, "You shall worship the Lord your God"? [Deuteronomy 6:13] Yet here it says, "fall down before His footstool:" and, explaining to us what His footstool is, it says, "The earth is My footstool." I am in doubt; I fear to worship the earth, lest He who made the heaven and the earth condemn me; again, I fear not to worship the footstool of my Lord, because the Psalm bids me, "fall down before His footstool." I ask, what is His footstool? And the Scripture tells me, "the earth is My footstool." In hesitation I turn unto Christ, since I am herein seeking Himself: and I discover how the earth may be worshipped without impiety, how His footstool may be worshipped without impiety. For He took upon Him earth from earth; because flesh is from earth, and He received flesh from the flesh of Mary. And because He walked here in very flesh, and gave that very flesh to us to eat for our salvation; and no one eats that flesh, unless he has first worshipped: we have found out in what sense such a footstool of our Lord's may be worshipped, and not only that we sin not in worshipping it, but that we sin in not worshipping. But does the flesh give life? Our Lord Himself, when He was speaking in praise of this same earth, said, "It is the Spirit that quickens, the flesh profits nothing."...But when our Lord praised it, He was speaking of His own flesh, and He had said, "Except a man eat My flesh, he shall have no life in him." [John 6:54] Some disciples of His, about seventy, were offended, and said, "This is an hard saying, who can hear it?" And they went back, and walked no more with Him. It seemed unto them hard that He said, "Except you eat the flesh of the Son of Man, you have no life in you:" they received it foolishly, they thought of it carnally, and imagined that the Lord would cut off parts from His body, and give unto them; and they said, "This is a hard saying." It was they who were hard, not the saying; for unless they had been hard, and not meek, they would have said unto themselves, He says not this without reason, but there must be some latent mystery herein. They would have remained with Him, softened, not hard: and would have learned that from Him which they who remained, when the others departed, learned. For when twelve disciples had remained with Him, on their departure, these remaining followers suggested to Him, as if in grief for the death of the former, that they were offended by His words, and turned back. But He instructed them, and says unto them, "It is the Spirit that quickens, but the flesh profits nothing; the words that I have spoken unto you, they are spirit, and they are life." [John 6:63] Understand spiritually what I have said; you are not to eat this body which you see; nor to drink that blood which they who will crucify Me shall pour forth. I have commended unto you a certain mystery; spiritually understood, it will quicken. Although it is needful that this be visibly celebrated, yet it must be spiritually understood.

[AD 430] Augustine of Hippo on Psalms 99:6-8
"Moses and Aaron among His priests, and Samuel among such as call upon His Name: these called upon the Lord, and He heard them" [Psalm 99:6]. "He spoke unto them out of the cloudy pillar" [Psalm 99:7]....Of Moses it is not there stated that he was a priest. But if he was not this, what was he? Could he be anything greater than a priest? This Psalm declares that he also was himself a priest: "Moses and Aaron among His priests." They therefore were the Lord's priests. Samuel is read of later in the Book of Kings: this Samuel is in David's times; for he anointed the holy David. Samuel from his infancy grew up in the temple....He mentions these: and by these desires us to understand all the saints. Yet why has he here named those? Because we said that we ought here to understand Christ. Attend, holy brethren. He said above, "O magnify the Lord our God: and fall down before His footstool, for He is holy:" praising some one, that is, our Lord Jesus Christ; whose footstool is to be worshipped, because He assumed flesh, in which He was to appear before the human race; and wishing to show unto us that the ancient fathers also had preached of Him, because our Lord Jesus Christ is Himself the True Priest, he mentioned these, because God spoke unto them out of the cloudy pillar. What means, "out of the cloudy pillar"? He was speaking figuratively. For if He spoke in some cloud, those obscure words predicted some one unknown, yet to be manifest. This unknown one is no longer unknown; for He is known by us, our Lord Jesus Christ....He who first spoke out of the cloudy pillar, has in Person spoken unto us in His footstool; that is, on earth, when He had assumed the flesh, for which reason we worship His footstool, for He is holy. He Himself used to speak out of the cloud, which was not then understood: He has spoken in His own footstool, and the words of His cloud have been understood. "They kept His testimonies, and the law that He gave them."..."You heard them," he says, "O Lord our God: You were forgiving to them, O God" [Psalm 99:8]. God is not said to be forgiving toward anything but sins: when He pardons sins, then He forgives. And what had He in them to punish, so that He was forgiving in pardoning them? He was forgiving in pardoning their sins, He was also forgiving in punishing them. For what follows? "And punished all their own affections." Even in punishing them You were forgiving toward them: for not in remitting, but also in punishing their sins, have You been forgiving. Consider, my brethren, what he has taught us here: attend. God is angry with him whom, when he sins, He scourges not: for unto him to whom He is truly forgiving, He not only remits sins, that they may not injure him in a future life; but also chastens him, that he delight not in continual sin.

[AD 430] Augustine of Hippo on Psalms 99:9
"O magnify the Lord our God!" [Psalm 99:9]. Again we magnify Him. He who is merciful even when He strikes, how is He to be praised, how is He to be magnified? Can you show this unto your son, and cannot God? For you are not good when thou dost caress your son, and evil when you strike him. Both when thou dost caress him you are a father, and when you strike him, you are his father: thou dost caress him, that he may not faint; you strike him, that he may not perish. "O magnify the Lord our God, and worship Him upon His holy hill: for the Lord our God is holy." As he said above, "O magnify the Lord our God and fall down before His footstool:" now we have understood what it is to worship His footstool: thus also but now after he had magnified the Lord our God, that no man might magnify Him apart from His hill, he has also praised His hill. What is His hill? We read elsewhere concerning this hill, that a stone was cut from the hill without hands, and shattered all the kingdoms of the earth, and the stone itself increased. This is the vision of Daniel which I am relating. This stone which was cut from the hill without hands increased, and "became," he says, "a great mountain, and filled the whole face of the earth." [Daniel 2:34-35] Let us worship on that great mountain, if we desire to be heard. Heretics do not worship on that mountain, because it has filled the whole earth; they have stuck fast on part of it, and have lost the whole. If they acknowledge the Catholic Church, they will worship on this hill with us. For we already see how that stone that was cut from the mountain without hands has increased, and how great tracts of earth it has prevailed over, and unto what nations it has extended. What is the mountain whence the stone was hewn without hands? The Jewish kingdom, in the first place; since they worshipped one God. Thence was hewn the stone, our Lord Jesus Christ....That stone then was born of the mountain without hands: it increased, and by its increase broke all the kingdoms of the earth. It has become a great mountain, and has filled the whole face of the earth. This is the Catholic Church, in whose communion rejoice that you are. But they who are not in her communion, since they worship and praise God apart from this same mountain, are not heard unto eternal life; although they may be heard unto certain temporal things. Let them not flatter themselves, because God hears them in some things: for He hears Pagans also in some things. Do not the Pagans cry unto God, and it rains? Wherefore? Because He makes His sun to rise over the good and the bad, and sends rain upon the just and the unjust. [Matthew 5:45] Boast not therefore, Pagan, that when you cry unto God, God sends rain, for He sends rain upon the just and the unjust. He has heard you in temporal things: He hears you not in things eternal, unless you have worshipped in His holy hill. "Worship Him upon His holy hill: for the Lord our God is holy."...