16 The Spirit itself beareth witness with our spirit, that we are the children of God:
[AD 253] Origen of Alexandria on Romans 8:16
The Spirit of adoption … bears witness and assures our spirits that we are children of God after we have passed from the spirit of slavery and come under the Spirit of adoption, when all fear has departed. We no longer act out of fear of punishment but do everything out of love for the Father. It is right too that the Spirit of God should be said to bear witness with our spirits and not with our souls, because the spirit is our better part.

[AD 258] Cyprian on Romans 8:16
Cyprian to the people abiding at Thibaris, greeting. I had indeed thought, beloved brethren, and prayerfully desired-if the state of things and the condition of the times permitted, in conformity with what you frequently desired-myself to come to you; and being present with you, then to strengthen the brotherhood with such moderate powers of exhortation as I possess. But since I am detained by such urgent affairs, that I have not the power to travel far from this place, and to be long absent from the people over whom by divine mercy I am placed, I have written in the meantime this letter, to be to you in my stead. For as, by the condescension of the Lord instructing me, I am very often instigated and warned, I ought to bring unto your conscience also the anxiety of my warning. For you ought to know and to believe, and hold it for certain, that the day of affliction has begun to hang over our heads, and the end of the world and the time of Antichrist to draw near, so that we must all stand prepared for the battle; nor consider anything but the glory of life eternal, and the crown of the confession of the Lord; and not regard those things which are coming as being such as were those which have passed away. A severer and a fiercer fight is now threatening, for which the soldiers of Christ ought to prepare themselves with uncorrupted faith and robust courage, considering that they drink the cup of Christ's blood daily, for the reason that they themselves also may be able to shed their blood for Christ. For this is to wish to be found with Christ, to imitate that which Christ both taught and did, according to the Apostle John, who said, "He that saith he abideth in Christ, ought himself also so to walk even as He walked." Moreover, the blessed Apostle Paul exhorts and teaches, saying, "We are God's children; but if children, then heirs of God, and joint-heirs with Christ; if so be that we suffer with Him, that we may also be glorified together."

[AD 258] Cyprian on Romans 8:16
Nor let anything now be revolved in your hearts and minds besides the divine precepts and heavenly commands, with which the Holy Spirit has ever animated you to the endurance of suffering. Let no one think of death, but of immortality; nor of temporary punishment, but of eternal glory; since it is written, "Precious in the sight of the Lord is the death of His saints; " and again, "A broken spirit is a sacrifice to God: a contrite and humble heart God doth not despise." And again, where the sacred Scripture speaks of the tortures which consecrate God's martyrs, and sanctify them in the very trial of suffering: "And if they have suffered torments in the sight of men, yet is their hope full of immortality; and having been a little chastised, they shall be greatly rewarded: for God proved them, and found them worthy of Himself. As gold in the furnace hath He tried them, and received them as a sacrifice of a burnt-offering, and in due time regard shall be had unto them. The righteous shall shine, and shall run to and fro like sparks among the stubble. They shall judge the nations, and have dominion over the people; and their Lord shall reign for ever." When, therefore, you reflect that you shall judge and reign with Christ the Lord, you must needs exult and tread under foot present sufferings, in the joy of what is to come; knowing that from the beginning of the world it has been so appointed that righteousness should suffer there in the conflict of the world, since in the beginning, even at the first, the righteous Abel was slain, and thereafter all righteous men, and prophets, and apostles who were sent. To all of whom the Lord also in Himself has appointed an example, teaching that none shall attain to His kingdom but those who have followed Him in His own way, saying, "He that loveth his life in this world shall lose it; and he that hateth his life in this world shall keep it unto life eternal." And again: "Fear not them which kill the body, but are not able to kill the soul: but rather fear Him who is able to destroy both soul and body in hell." Paul also exhorts us that we who desire to attain to the Lord's promises ought to imitate the Lord in all things. "We are," says he, "the sons of God: but if sons, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with Him, that we may also be glorified together." Moreover, he added the comparison of the present time and of the future glory, saying, "The sufferings of this present time are not worthy to be compared with the coming glory which shall be revealed in us." Of which brightness, when we consider the glory, it behoves us to bear all afflictions and persecutions; because, although many are the afflictions of the righteous, yet those are delivered from them all who trust in God.

[AD 258] Cyprian on Romans 8:16
That we must press on and persevere in faith and virtue, and in completion of heavenly and spiritual grace, that we may attain to the palm and the crown. In the book of Chronicles: "The Lord is with you so long as ye also are with Him; but if ye forsake Him, He will forsake you." In Ezekiel also: "The righteousness of the righteous shall not deliver him in what day soever he may transgress." Moreover, in the Gospel the Lord speaks, and says: "He that shall endure to the end, the same shall be saved." And again: "If ye shall abide in my word, ye shall be my disciples indeed; and ye shall know the truth, and the truth shall make you free." Moreover, forewarning us that we ought always to be ready, and to stand firmly equipped and armed, He adds, and says: "Let your loins be girded about, and your lamps burning, and ye yourselves like unto men that wait for their lord when he shall return from the wedding, that when he cometh and knocketh they may open unto him. Blessed are those servants whom their lord, when he cometh, shall find watching." Also the blessed Apostle Paul, that our faith may advance and grow, and attain to the highest point, exhorts us, saying: "Know ye not, that they which run in a race run all indeed, yet one receiveth the prize? So run, that ye may obtain. And they, indeed, that they may receive a corruptible crown; but ye an incorruptible." And again: "No man that warreth for God binds himself to anxieties of this world, that he may be able to please Him to whom he hath approved himself. Moreover, also, if a man should contend, he will not be crowned unless he have fought lawfully." And again: "Now I beseech you, brethren, by the mercy of God, that ye constitute your bodies a living sacrifice, holy, acceptable unto God; and be not conformed to this world, but be ye transformed in the renewing of your spirit, that ye may prove what is the will of God, good, and acceptable, and perfect." And again: "We are children of God: but if children, then heirs; heirs indeed of God, but joint-heirs with Christ, if we suffer together, that we may also be glorified together." And in the Apocalypse the same exhortation of divine preaching speaks, saying, "Hold fast that which thou hast, lest another take thy crown; " which example of perseverance and persistence is pointed out in Exodus, when Moses, for the overthrow of Ama-lek, who bore the type of the devil, raised up his open hands in the sign and sacrament of the cross, and could not conquer his adversary unless when he had stedfastly persevered in the sign with hands continually lifted up. "And it came to pass," says he, "when Moses raised up his hands, Israel prevailed; but when he let down his hands, Amalek grew mighty. And they took a stone and placed it under him, and he sate thereon. And Aaron and Hur held up his hands on the one side and on the other side, and Moses' hands were made steady even to the going down of the sun. Anti Jesus routed Amalek and all his people. And the Lord said unto Moses, Write this, and let it be a memorial in a book, and tell it in the ears of Jesus; because in destroying I will destroy the remembrance of Amalek from under heaven."

[AD 258] Cyprian on Romans 8:16
Of the benefits of martyrdom. In the Proverbs of Solomon: "The faithful martyr delivers his soul from evils." Also in the same place: "Then shall the righteous stand in great boldness against them who have afflicted them, and who took away their labours. When they see them, they shall be disturbed with a horrible fear; and they shall wonder at the suddenness of their unhoped-for salvation, saying among themselves, repenting and groaning with distress of spirit, These are they whom some time we had in derision, and in the likeness of a proverb; we fools counted their life madness, and their end without honour. How are they reckoned among the children of God, and their lot among the saints! Therefore we have wandered from the way of truth, and the light of righteousness has not shined upon us, and the sun has not risen upon us. We have been wearied in the way of iniquity and of perdition, and we have walked through difficult solitudes; but we have not known the way of the Lord. What hath pride profited us? or what hath the boasting of riches brought to us? All these things have passed away as a shadow." Of this same thing in the cxvth Psalm: "Precious in the sight of the Lord is the death of His saints." Also in the cxxvth Psalm: "They who sow in tears shall reap in joy. Walking they walked, and wept as they cast their seeds; but coming they shall come in joy, raising up their laps." Of this same thing in the Gospel according to John: "He who loveth his life shall lose it; and he that hateth his life in this world shall find it to life eternal." Also in the same place: "But when they shall deliver you up, take no thought what ye shall speak; for it is not ye who speak, but the Spirit of your Father which speaketh in you." Also in the same place: "The hour shall come, that every one that killeth you shall think he doeth service to God l but they shall do this also because they have not known the Father nor me." Of this same matter, according to Matthew: "Blessed are they which shall suffer persecution for righteousness' sake; for theirs is the kingdom of heaven." Also in the same place: "Fear not them which kill the body, but are not able to kill the soul; but rather fear Him which is able to kill the soul and body in Gehenna." Also in the same place: "Whosoever shall confess me before men, him also will I confess before my Father which is in heaven; but he who shall deny me before men, him also will I deny before my Father which is in heaven. And he that shall endure to the end, the same shall be saved." Of this same thing, according to Luke: "Blessed shall ye be when men shall hate you, and shall separate you (from their company), and shall drive you out, and shall speak evil of your name, as wicked, for the Son of man's sake. Rejoice in that day, and exult; for, lo, your reward is great in heaven." Also in the same place: "Verily I say unto you, There is no man that leaveth house, or parents, or brethren, or wife, or children, for the sake of the kingdom of God, and does not receive seven times as much in this present time, but in the world to come life everlasting." Of this same thing in the Apocalypse: "And when he had opened the fifth seal, I saw under the altar of God the souls of them that were slain on account of the word of God and His testimony. And they cried with a loud voice, saying, How long, O Lord, holy and true, dost Thou not judge and avenge our blood on them that dwell on the earth? And unto every one of them were given white robes; and it was said to them, that they should rest still for a short time, until the number of their fellow-servants, and of their brethren, should be fulfilled, and they who shall afterwards be slain, after their example." Also in the same place: "After these things I saw a great crowd, which no one among them could number, from every nation, and from every tribe, and from every people and tongue, standing before the throne and before the Lamb; and they were clothed with white robes, and palms were in their hands. And they said with a loud voice, Salvation to our God, that sitteth upon the throne, and to the Lamb. And one of the elders answered and said to me, What are these which are clothed with white robes? who are they, and whence have they come? And I said unto him, My lord, thou knowest. And he said unto me, These are they who have come out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb. Therefore they are before the throne of God, and serve Him day and night in His temple; and He who sitteth upon the throne shall dwell among them. They shall neither hunger nor thirst ever; and neither shall the sun fall upon them, nor shall they suffer any heat: for the Lamb who is in the midst of the throne shall protect them, and shall lead them to the fountains of the waters of life; and God shall wipe away every tear from their eyes." Also in the same place: "He who shall overcome I will give him to eat of the tree of life, which as in the paradise of my God." Also in the same place: "Be thou faithful even unto death, and I will give thee a crown of life." Also in the same place: "Blessed shall they be who shall watch, and shall keep their garments, lest they walk naked, and they see their shame." Of this same thing, Paul in the second Epistle to Timothy: "I am now offered up, and the time of my assumption is at hand. I have fought a good fight, I have finished my course, I have kept the faith. There now remains for me a crown of righteousness, which the Lord, the righteous Judge, will give me in that day; and not only to me, but to all also who love His appearing." Of this same thing to the Romans: "We are the sons of God: but if sons and heirs of God, we are also joint-heirs with Christ; if we suffer together, that we may also be magnified together." Of this same thing in the cxviiith Psalm: "Blessed are they who are undefiled in the way, and walk in the law of the Lord. Blessed are they who search into His testimonies."

[AD 384] Ambrosiaster on Romans 8:16
The witness of children is that by the Spirit they should be seen to bear the sign of the Father.

[AD 390] Diodorus of Tarsus on Romans 8:16
Paul showed by this that he called the soul “spirit” when it was spiritual, and the gift of the Spirit “Spirit.”

[AD 407] John Chrysostom on Romans 8:16
For it is not from the language merely, he says, that I make my assertion, but from the cause out of which the language has its birth; since it is from the Spirit suggesting it that we so speak. And this in another passage he has put into plainer words, thus: "God has sent forth the Spirit of his Son into our hearts, crying, Abba Father." [Galatians 4:6] And what is that, "Spirit bears witness with spirit?" The Comforter, he means, with that Gift, which is given unto us. For it is not of the Gift alone that it is the voice, but of the Comforter also who gave the Gift, He Himself having taught us through the Gift so to speak. But when the "Spirit bears witness" what farther place for doubtfulness? For if it were a man, or angel, or archangel, or any other such power that promised this, then there might be reason in some doubting. But when it is the Highest Essence that bestows this Gift, and "bears witness" by the very words He bade us use in prayer, who would doubt any more of our dignity? For not even when the Emperor elects any one, and proclaims in all men's hearing the honor done him, does anybody venture to gainsay.

[AD 407] John Chrysostom on Romans 8:16
This means that the Comforter bears witness with the gift which he has given us. For it is not merely the gift which speaks but the Comforter who gave the gift as well.

[AD 418] Pelagius on Romans 8:16
The evidence of our adoption is that we have the Spirit, through whom we pray in the manner mentioned above; for only sons could receive such a pledge.

[AD 458] Theodoret of Cyrus on Romans 8:16
Paul uses the word spirit in two senses. The first is the Spirit of God, the second is our spirit, i.e., through the grace which we have been given.