Why didn't Jesus inherit Mary's sin nature?

We should first acknowledge that Mary, the mother of Jesus, was indeed a sinner (cf. Rom. 3:23). Mary's own personal testimony was that she was a sinner, when she declared "My soul magnifies the Lord, and my spirit rejoices in God my Savior" (Luke 1:46-47), calling Jesus "my Savior" was confessing her need of salvation from sin.

At the same time, Scripture clearly says Jesus was / is sinless (Isa. 53:9-11; 2 Cor. 5:21; Heb. 4:15; 7:26; 1 Pet. 2:22, 1 John 3:5). And if he had ever sinned, he would be disqualified from being the only redeemer of God's elect (WSC Q. 21). But the Bible tells us Jesus saved his people from their sin (Matt. 1:21; 1 Tim. 4:10; cf. Acts 4:12).

So then, how can a sinful woman give birth to a sinless son? The answer is in the virgin birth. According to the Westminster Shorter Catechism:

Christ, the Son of God, became man, by taking to himself a true body and a reasonable soul, being conceived by the power of the Holy Ghost, in the womb of the Virgin Mary, and born of her, yet without sin. (Q. 22)

Mary was a virgin (Luke 1:34; cf. Matt. 1:25). She was found to be with child, not from being with a man, but by the Holy Spirit (Matt. 1:18; cf. Luke 1:35): "for that which is conceived in her is from the Holy Spirit" (Matt. 1:20). Because Jesus' birth was brought about from above, Mary's sin was by-passed and there was given to the world the Christ, the sinless Son of the living God (Matt. 16:16; John 3:16). Jesus, the second and last man Adam (1 Cor. 15:45, 47), did not inherit a sin nature (cf. Rom. 5:12-21).

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