5 Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?
"But the very hairs of your head are all numbered," says the Lord; those on the chin, too, are numbered, and those on the whole body. There must be therefore no plucking out, contrary to God's appointment, which has counted them in according to His will. "Know ye not yourselves," says the apostle, "that Christ Jesus is in you?" Whom, had we known as dwelling in us, I know not how we could have dared to dishonour.
Paul is saying this because if we do not know how to put one another to the test, we do not know whether Christ is in us or not. To fail to meet the test is not to know the faith inherent in our religious profession. A person who has a sense of faith in his heart knows that Jesus Christ is within him. Commentary on Paul’s Epistles.
"Try your own selves, whether ye be in the faith, prove your own selves. Know ye not as to your own selves, that Christ is in you, unless indeed ye be reprobate?" For since by what he has said he hath shown that even if he does not punish, it is not because he hath not Christ in himself, but because he intimates His long-suffering, Who was crucified and yet avenged not Himself; he again, in another manner, produces the same effect, and still more irrefragably, establishing his argument by the disciples.
"For why speak I of myself," he says, "the teacher, who have so much care upon me and am entrusted with the whole world and have done such great miracles. For if ye will but examine yourselves who are in the rank of disciples, ye will see that Christ is in you also. But if in you, then much more in your teacher. For if ye have faith, Christ is in you also." For they who then believed wrought miracles. Wherefore also he added, "Try your own selves, prove your own selves, whether ye be in the faith. Know ye not as to your own selves, unless indeed that Christ is in you, ye be reprobate?"
He seems to me here to speak of the "faith" which relates to miracles. "For if ye have faith," he says, "Christ is in you, except ye have become reprobates." Seest thou how again he terrifies them, and shows even to superfluity that Christ is with Him. For he seems to me to be here alluding to them, even as to their lives. For since faith is not enough [by itself] to draw down the energy of the Spirit, and he had said that "if ye are in the faith" ye have Christ in you, and it happened that many who had faith were destitute of that energy; in order to solve the difficulty, he says, "except ye be reprobate," except [that is] ye are corrupt in life.
And what am I saying about myself, that Christ speaks in me, that I am powerful, even though I imitate Him who was crucified and do not proceed to punishment. For you too, if you wish to test yourselves, will see Christ in yourselves, since you all have faith — either ordinary faith, or in particular — wonder-working faith (for believers at that time performed miracles). Thus, if Christ is in you — my disciples — then all the more so in me, the teacher.
That is, if you have not been deprived of the gift of miracles which you received. He hints that they are living lawlessly, for many, having faith, did not have the power to work miracles, because they lived lawlessly. The meaning of the words is as follows: you, it seems, do not know yourselves — whether Christ is in you? He is not in you, because you are "not what you ought to be." But he expresses this thought not directly, but speaks indefinitely, so as not to strike them.
[AD 215] Clement of Alexandria on 2 Corinthians 13:5