For I through the Law died unto the Law. This may be viewed in two ways; it is either the law of grace which he speaks of, for he is wont to call this a law, as in the words for the law of the Spirit of life made me free (Rom 8:2).
Or it is the old Law, of which he says, that by the Law itself he has become dead to the Law. That is to say, the Law itself has taught me no longer to obey itself, and therefore if I do so, I shall be transgressing even its teaching.
[...] Having said I am dead, lest it should be objected, how then dost thou live? he adds the cause of his living, and shows that when alive the Law slew him, but that when dead Christ through death restored him to life.
He shows the wonder to be twofold; that by Christ both the dead was begotten into life, and that by means of death. He here means the immortal life, for this is the meaning of the words, That I might live unto God I am crucified with Christ.
[...] In the words I am crucified with Christ he alludes to Baptism, and in the words nevertheless I live, yet not I he alludes to our subsequent manner of life whereby our members are mortified.
By saying Christ liveth in me he means nothing is done by me, which Christ disapproves. For as by death he signifies not what is commonly understood, but a death to sin; so by life, he signifies a delivery from sin.
For a man cannot live to God, otherwise than by dying to sin; and as Christ suffered bodily death, so does Paul a death to sin. Mortify, says he your members which are upon the earth; fornication, uncleanness, passion (Col. 3:5), and again our old man was crucified (Rom. 6:6) which took place in Baptism.
[...] He says not I live, but Christ liveth in me. Who is bold enough to utter such words?
Paul indeed, who had harnessed himself to Christ’s yoke, and cast away all worldly things, and was paying universal obedience to His will, says not I live to Christ, but, what is far higher, Christ liveth in me.
As sin, when it has the mastery, is itself the vital principle, and leads the soul whither it will, so, when sin is slain, and when the will of Christ is obeyed, this life is no longer earthly, but Christ liveth, that is, works – has mastery – within us.
John Chrysostom (c.347-407): Commentary on Galatians, c.2.