19 Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven.
[AD 253] Origen of Alexandria on Matthew 18:18-20
Let us look well whether this precept extends to all sin; for what if any one sin any of those sins which are unto death, such as unnatural crimes, adultery, homicide, or effeminacy, it cannot be meant that such as these are to be admonished privately, and if he hear you, forthwith to say that you have gained him. And not rather first put him out of the Church, or only when remaining obstinate after monition before witnesses, and by the Church? One man, looking at the infinite mercy of Christ, will say, that since the words of Christ make no distinction of sins, it is to go against Christ's mercy to limit His words only to little sins. Another, on the other hand, considering the words carefully, will aver, that they are not spoken of every sin; for that he that is guilty of those great sins is not a brother, but is called a brother, with whom, according to the Apostle, we ought not so much as to eat. But as they who expound this as referring to every sin give encouragement to the careless to sin; so, on the other hand, he, who teaches that one having sinned in little sins and such as are not deadly, is, when he has spurned the admonition of the witnesses and the Church, to be held as a heathen and a publican, seems to introduce too great severity. For whether he finally perishes, we are not able to decide. First, because he who has been thrice told of his fault and not hearkened, may hearken the fourth time; secondly, because sometimes a man does not receive according to his deeds, but beyond his trespass, which is good for him in this world; lastly, because He said not alone, Let him be as a heathen, but Let him be to thee. Whosoever then when reproved three times in a light trespass, does not amend, him we ought to hold for a heathen and a publican, avoiding him, that he may be brought to confusion. But whether he is esteemed of God also as a heathen and a publican, is not ours to decide, but is in the judgment of God.

He said not in the heavens (in cœlis), as when He spoke to Peter, but in heaven (in cœlo), for they are not yet attained to the like perfection with Peter.

And He said not, I will be, but I am in the midst of them; because straightway, as soon as they have agreed together, Christ is found among them.

And this also is the reason why our prayers are not granted, because we do not agree together in all things upon earth, neither in doctrine, nor in conversation. For as in music, unless the voices are in time there is no pleasure to the hearer, so in the Church, unless they are united God is not. pleased therein, nor does He hear their words.

Or, In whatever the two testaments are in agreement, for this every prayer is found acceptable to God.

[AD 367] Hilary of Poitiers on Matthew 18:18-20
To hold out a great and terrible fear, by which all men should be reached in this present life, He pronounces that the judgment of the Apostles should be ratified, so that whosoever they bound on earth, i. e. left entangled in the noose of sin, and whosoever they loosed, i. e. accorded the pardon of God's mercy to their salvation, that these should be bound and loosed in heaven.

For He who is peace and charity, will set His place and habitation in good and peaceable dispositions.

[AD 407] John Chrysostom on Matthew 18:18-20
And be it noted, that He said not to the Primate1 of the Church, Bind such a man; but, If ye shall bind him, the bonds shall be indissoluble; leaving the other to his discretion. And see how He has set the incorrigible person under the yoke of a twofold necessity; to wit, the punishment that is here, namely, the casting forth out of the Church, when He said, Let him be to thee as a heathen; and the future punishment, saying, that he shall be bound in heaven; thus by the weight of his penalties lessening his brother's wrath against him.

Or, because He had said, It shall be done unto them by My Father; therefore, to show that He is the Giver together with His Father, He adds this, where two or three, &c.

Yet He said not barely, Where they are gathered together, but added, in my name, as much as to say, If any man look upon Me as the chief motive of his love to his neighbour, I will be with him, though he is virtue be shown towards other men. How is it then that those who thus agree together do not obtain what they ask for? First, because they ask things not expedient, and because they do not bring on their parts that which they ought to contribute; wherefore He says, If two of you, that is, who show an evangelic conversation. Thirdly, because they pray seeking vengeance against those who have grieved them. And fourthly, because they seek mercy for sinners who have not repented.

[AD 420] Jerome on Matthew 18:18-20
Because He had said, If he will not hear the Church, let him be to thee as a heathen, and a publican, whereupon the brother so contemned might answer, or think within himself, If you despise me, I also will despise you; if you condemn me, you shall be condemned by my sentence. He therefore confers powers upon the Apostles, that they may be assured that when any are condemned after this manner, the sentence of man is ratified by the sentence of God. Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven; and whatsoever ye shall loose upon the earth shall be loosed in heaven.

Or otherwise; All His foregoing discourse had invited us to union; now to make us embrace peace more anxiously, He holds out a reward, promising to be in the midst of two or three.

(vid. Origen. in loc.) We may also understand this spiritually; where our spirit, soul, and body are in agreement, and have not within them conflicting wills, they shall obtain from My Father every thing they shall ask; for none can doubt that that demand is good, where the body wills the same thing as the spirit.

[AD 430] Augustine of Hippo on Matthew 18:18-20
(ubi sup.) Otherwise; When you begin to hold your brother as a publican you bind him on earth, but take heed that you bind him with just cause; for an unjust cause breaks rightful bonds. But when you have corrected him, and agreed with him, you have loosed him upon earth, and when you have loosed him upon earth, he shall be loosed also in heaven. You confer a great boon not on yourself, but on him, as he had done the hurt not to you but to himself.

[AD 1274] Glossa Ordinaria on Matthew 18:18-20
(ap. Anselm.) But He holds out a ratification not only of sentences of excommunication, but of every petition which is offered by men holding together in the unity of the Church; for He adds, Again I say unto you, that if two of you shall agree upon earth, whether in admitting a penitent, or casting out a forward person, touching any thing which they shall ask, any thing, that is, that is not against the unity of the Church, it shall be done for them by my Father which is in heaven. By saying, which is in heaven, He points Him out as above all, and therefore able to fulfil all that shall be asked of Him. Or, He is in the heavens, that is, with saints, proof enough that whatever worthy thing they shall ask shall be done unto them, because they have with them Him of whom they ask. For this cause is the sentence of those that agree together ratified, because God dwells in them, For where two or three are gathered together in my name, there am I in the midst of them.

[AD 253] Origen of Alexandria on Matthew 18:19
Concord occurs in two kinds: first, in agreement of thought, when two minds think the same ideas (as the apostle called it) and have the same thoughts. Second, in agreement of will, in living similar lives. But since “If two of you agree on earth about anything they ask, it will be done for them by Jesus’ Father in heaven,” if something is asked of the Father in heaven and is not granted, it is clear that the two here on earth are not in harmony. The reason why we do not achieve our desires and prayers is our own fault: we do not agree either in our thoughts or in our way of life. Besides, if we are the body of Christ and “God arranged the members in the body, each one of them … so that the members may have the care for one another, and if one member suffers all suffer together; if one member is honored all rejoice together,” then we must practice the harmony that comes from God’s music, so that when we are gathered in Christ’s name, Christ may be in our midst, who is God’s word and wisdom and power.

[AD 406] Chromatius of Aquileia on Matthew 18:19
How important a place the unanimity and harmony of brothers hold with God! We can know from this very fact that the Lord has clearly said that when two or three pray in unanimity, the Father grants everything from heaven. For with God nothing is more pleasing than brotherly peace, nothing better than unanimity and harmony, according to what is written: “Behold, how good and pleasant it is when brothers dwell in unity. And again: “There is great peace among those who love your name, and there is no stumbling block for them.” And in another place: “The God who makes us live in harmony in the house.” So Isaiah also testified: “Lord,” he said, “our God, give us peace. For you have given us everything.” That this harmony of brothers is pleasing to God, the Holy Spirit declared through Solomon, saying, “There are three things which are pleasing to God and men: harmony of brothers, love of one’s neighbors and the union of man and woman.” So quite rightly the Lord in this passage testifies that when two or three come together on earth, whatever they ask for, all is granted by the Father.

[AD 407] John Chrysostom on Matthew 18:19
Do you see how by another motive also He puts down our enmities, and takes away our petty dissensions, and draws us one to another, and this not from the punishment only which has been mentioned, but also from the good things which spring from charity? For having denounced those threats against contentiousness, He puts here the great rewards of concord, if at least they who are of one accord do even prevail with the Father, as touching the things they ask, and have Christ in the midst of them.

Are there then indeed nowhere two of one accord? Nay, in many places, perchance even everywhere. How then do they not obtain all things? Because many are causes of their failing. For either they often ask things inexpedient. And why do you marvel, if this is the case with some others, whereas it was so even with Paul, when he heard, My grace is sufficient for you; for my strength is perfected in weakness. Or they are unworthy to be reckoned with them that heard these words, and contribute not their own part, but He seeks for such as are like them; therefore He says of you, of the virtuous, of them that show forth an angelic rule of life. Or they pray against them that have aggrieved them, seeking for redress and vengeance; and this kind of thing is forbidden, for, Pray, says He, for your enemies. Matthew 5:44 Or having sins unrepented they ask mercy, which thing it is impossible to receive, not only if themselves ask it, but although others having much confidence towards God entreat for them, like as even Jeremiah praying for the Jews did hear, Pray not thou for this people, because I will not hear you. Jeremiah 11:14

But if all things are there, and thou ask things expedient, and contribute all your own part, and exhibit an life, and have concord and love towards your neighbor, you will obtain on your entreaty; for the Lord is loving towards man.
[AD 420] Jerome on Matthew 18:19-20
(Verse 19, 20.) Again I say to you: if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I among them. The above discourse has encouraged us towards harmony. Therefore, it promises a reward so that we may hasten eagerly towards peace, as it says that it will be present among two or three. Similar to the example of a tyrant, who wanted to prove that he had captured two friends (when one returned to see his mother and gave his friend as a surety for himself), so he wanted to test one, and release the other. And when he returned on the appointed day, marveling at the faith of both, he asked that they have a third. We can also understand this spiritually, that where the spirit and soul and body agree and do not have war between themselves of different desires (or pleasures); with the flesh desiring against the spirit, and the spirit against the flesh, they obtain from the Father whatever they seek about any thing. And there is no doubt that the request of good things is certain, where the body wants to have those things which the spirit wants.