4 But the apostle himself also, being one who had been formed in a womb, and had issued thence, wrote to us, and confessed in his Epistle to the Philippians that “to live in the flesh was the fruit of [his] work;” thus expressing himself. Now the final result of the work of the Spirit is the salvation of the flesh. For what other visible fruit is there of the invisible Spirit, than the rendering of the flesh mature and capable of incorruption? If then [he says], “To live in the flesh, this is the result of labour to me,” he did not surely contemn the substance of flesh in that passage where he said, “Put off the old man with his works;” but he points out that we should lay aside our former conversation, that which waxes old and becomes corrupt; and for this reason he goes on to say, “And put on the new man, that which is renewed in knowledge, after the image of Him who created him.”
In this, therefore, that he says, “which is renewed in knowledge,” he demonstrates that he, the selfsame man who was in ignorance in times past, that is, in ignorance of God, is renewed by that knowledge which has respect to Him. For the knowledge of God renews man. And when he says, “after the image of the Creator,” he sets forth the recapitulation of the same man, who was at the beginning made after the likeness of God.
Source: Adversus haereses (New Advent)