Adam Clarke's Commentary on the Bible

Verse 17 (2 Corinthians 5:17)

If any man be in Christ, he is a new creature - It is vain for a man to profess affinity to Christ according to the flesh, while he is unchanged in his heart and life, and dead in trespasses and sins; for he that is in Christ, that is, a genuine Christian, having Christ dwelling in his heart by faith, is a new creature; his old state is changed: he was a child of Satan, he is now a child of God; he was a slave of sin, and his works were death; he is now made free from sin, and has his fruit unto holiness, and the end everlasting life. He was before full of pride and wrath; he is now meek and humble. He formerly had his portion in this life, and lived for this world alone; he now hath God for his portion, and he looks not at the things which are seen, but at the things which are eternal. Therefore, old things are passed away.

Behold, all things are become new - The man is not only mended, but he is new made; he is a new creature, καινη κτισις, a new creation, a little world in himself; formerly, all was in chaotic disorder; now, there is a new creation, which God himself owns as his workmanship, and which he can look on and pronounce very good. The conversion of a man from idolatry and wickedness was among the Jews denominated a new creation. He who converts a man to the true religion is the same, says R. Eliezer, as if he had created him.

- Adam Clarke's Commentary on the Bible