Augustine:

1) Now this resurrection regards not the body, but the soul. For souls, too, have a death of their own in wickedness and sins, whereby they are the dead of whom the same lips say, “Suffer the dead to bury their dead,”–that is, let those who are dead in soul bury them that are dead in body. It is of these dead, then–the dead in ungodliness and wickedness–that He says, “The hour is coming, and now is, when the dead shall hear the voice of the Son of God; and they that hear shall live.” “They that hear,” that is, they who obey, believe, and persevere to the end. Here no difference is made between the good and the bad. For it is good for all men to hear His voice and live, by passing to the life of godliness from the death of ungodliness. Of this death the Apostle Paul says, “Therefore all are dead, and He died for all, that they which live should not henceforth live unto themselves, but unto Him which died for them and rose again.” Thus all, without one exception, were dead in sins, whether original or voluntary sins, sins of ignorance, or sins committed against knowledge; and for all the dead there died the one only person who lived, that is, who had no sin whatever, in order that they who live by the remission of their sins should live, not to themselves, but to Him who died for all, for our sins, and rose again for our justification, that we, believing in Him who justifies the ungodly, and being justified from ungodliness or quickened from death, may be able to attain to the first resurrection which now is. For in this first resurrection none have a part save those who shall be eternally blessed; but in the second, of which He goes on to speak, all, as we shall learn, have a part, both the blessed and the wretched. The one is the resurrection of mercy, the other of judgment. And therefore it is written in the psalm, “I will sing of mercy and of judgment: unto Thee, O Lord, will I sing.” Augustine,“The City of God and Christian Doctrine,” in The Nicene and Post-Nicene Fathers, First Series. 2:245.

2) Likewise to the Corinthians he says: “For I delivered to you first of all that which I also received, how that Christ died for our sins according to the Scriptures.” Again, in his Second Epistle to these Corinthians: “For the love of Christ constrains us; because we thus judge, that if One died for all, then all died: and for all did Christ die, that they which live should no longer live unto themselves, but unto Him which died for them, and rose again. Wherefore, henceforth know we no man after the flesh; yea, though we have known Christ after the flesh, yet from henceforth know we Him so no more. Therefore if any man be in Christ, he is a new creature; old things are passed away; behold, all things are become new. And all things are of God, who hath reconciled us to Himself by Jesus Christ, and hath given unto us the minis try of reconciliation. To what effect? That God was in Christ, reconciling the world unto Himself, not imputing their trespasses unto them, and putting on us the ministry of reconciliation. Now then are we ambassadors for Christ, as though God did beseech you by us; we pray you in Christ’s stead, to be reconciled to God. For He hath made Him to be sin for us, who knew no sin; that we might become the righteousness of God in Him. We then, as workers together with Him, beseech you also that ye receive not the grace of God in vain. (For He saith, I have heard thee in an acceptable time, and in the day of salvation have I succored thee: behold, now is the acceptable time; behold, now is the day of salvation.)” Now, if infants are not embraced within this reconciliation and salvation, who wants them for the baptism of Christ? But if they are embraced, then are they reckoned as among the dead for whom He died; nor can they be possibly reconciled and saved by Him, unless He remit and impute not unto them their sins. Augustine, “Anti-Pelagian Writings,” in The Nicene and Post-Nicene Fathers, First Series, 5:32.

3) 8. The context of the passage containing this clause of which our adversaries make such a bad use, brings out its real meaning. “The love of Christ,” we read, “constrains us, because we thus judge, that if one died for all, then all died; and He died for all, that they which live should not henceforth live unto themselves, but to Him who died for them, and rose again. Therefore henceforth know we no man after the flesh; and though we have known Christ after the flesh, yet now henceforth know we Him no more.” The words, “that they which live should not henceforth live unto themselves, but unto Him who died for them, and rose again,” show plainly that the resurrection of Christ is the ground of the apostle’s statement. To live not to themselves, but to Him, must mean to live not after the flesh, in the hope of earthly and perishable goods, but after the spirit, in the hope of resurrection,–a resurrection already accomplished in Christ. Of those, then, for whom Christ died and rose again, and who live henceforth not to themselves, but to Him, the Apostle says that he knows no one after the flesh, on account of the hope of future immortality to which they were looking forward,–a hope which in Christ was already a reality. So, though he has known Christ after the flesh, before His death, now he knows Him no more; for he knows that He has risen, and that death has no more dominion over Him. And because in Christ we all are even now in hope, though not in reality, what Christ is, he adds: “Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. And all things are of God, who has reconciled us to Himself by Christ.” What the new creature–that is, the people renewed by faith–hopes for regarding itself, it has already in Christ; and the hope will also hereafter be actually realized. And, as regards this hope, old things have passed away, because we are no longer in the times of the Old Testament, expecting a temporal and carnal kingdom of God; and all things are become new, making the promise of the kingdom of heaven, where there shall be no death or corruption, the ground of our confidence. But in the resurrection of the dead it will not be as a matter of hope, but in reality, that old things shall pass away, when the last enemy, death, shall be destroyed; and all things shall become new when this corruptible has put on incorruption, and this mortal has put on immortality. This has already taken place in Christ, whom Paul accordingly, in reality, knew no longer after the flesh. But not yet in reality, but only in hope, did be know no one after the flesh of those for whom Christ died and rose again. For, as he says to the Ephesians, we are already saved by grace. The whole passage is to the purpose: “But God, who is rich in mercy, for His great love wherewith He loved us, even when we were dead in sins, hath quickened us together with Christ, by whose grace we have been saved.” The words, “hath quickened us together with Christ,” correspond to what he said to the Corinthians, “that they which live should no longer live to themselves, but to Him that died for them and rose again.” And in the words, “by whose grace we have been saved,” he speaks of the thing hoped for as already accomplished. So, in the passage quoted above, he says explicitly, “We have been saved by hope.” And here he proceeds to specify future events as if already accomplished. “And has raised us up together,” he says, “and has made us sit together in heavenly places in Christ Jesus.” Christ is certainly already seated in heavenly places, but we not yet. But as in an assured hope we already possess the future, he says that we sit in heavenly places, not in ourselves, but in Him. And to show that it is still future, in case it should be thought that what is spoken of as accomplished in hope has been accomplished in reality, he adds, “that He might show in the ages to come the exceeding riches of His grace in His kindness towards us in Christ Jesus.” So also we must understand the following passage: “For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death.” He says, “when we were in the flesh,” as if they were no longer in the flesh. He means to say, when we were in the hope of fleshly things, referring to the time when the law, which can be fulfilled only by spiritual love, was in force, in order that by transgression the offense might abound, that after the revelation of the New Testament, grace and the gift by grace might much more abound. And to the same effect he says elsewhere, “They which are in the flesh cannot please God;” and then, to show that he does not mean those not yet dead, he adds, “But ye are not in the flesh, but in the Spirit.” The meaning is, those who are in the hope of fleshly good cannot please God; but you are not in the hope of fleshly things, but in the hope of spiritual things, that is, of the kingdom of heaven, where the body itself, which now is natural, will, by the change in the resurrection, be, according to the capacity of its nature, a spiritual body. For “it is sown a natural body, it will be raised a spiritual body.” If, then, the apostle knew no one after the flesh of those who were said to be not in the flesh, because they were not in the hope of fleshly things, although they still were burdened with corruptible and mortal flesh; how much more significantly could he say of Christ that he no longer knew Him after the flesh, seeing that in the body of Christ what they hoped for had already been accomplished! Surely it is better and more reverential to examine the passages of sacred Scripture so as to discover their agreement with one another, than to accept some as true, and condemn others as false, whenever any difficulty occurs beyond the power of our weak intellect to solve. As to the apostle in his childhood understanding as a child, this is said merely as an illustration. And when he was a child he was not a spiritual man, as he was when he produced for the edification of the churches those writings which are not, as other books, merely a profitable study, but which authoritatively claim our belief as part of the ecclesiastical canon. Augustine, ‘The Writings Against the Manichæans and Against the Donatists,” in The Nicene and Post-Nicene Fathers, First Series, 4:181-183.