A non-Christian asked if we teach that Jesus was sinless because he did not have a human father, wasn't his human mother, Mary, sinful? Why wasn't her sin passed on to her son, Jesus?
You ask an important question about the sinlessness and perfection of Jesus Christ.
As you suggest, if there is any way in which Christ partakes of sin, he is disqualified from being the only redeemer of God’s elect (Westminster Shorter Catechism, Q. 21). The testimony of Scripture about this is clear. Hebrews says that Christ was tempted in all points as we are, yet without sinning (Heb. 4:15). Christ challenged his adversaries to prove that he had sinned and they could not (John 8:46). As the apostle Paul put it, “For our sake he made him [Christ] to be sin, who knew no sin, so that in him we might become the righteousness of God” (2 Cor. 5:21, cf. 1 Pet. 2:22, 1 John 3:5). He is a high priest unlike any other who is “holy, innocent, unstained, separated from sinners, and exalted above the heavens” (Heb. 7:26). His title as the Lamb of God (John 1:29) also draws our attention to his innocence.
The early church thought deeply about this as it formulated clear statements about Jesus Christ having two natures, a divine nature and a human nature, yet being one person. Christ unites in himself uniquely and in an unrepeatable way, this union (called the hypostatic union) of the human and the divine. This is the mystery of the Incarnation that the creeds of the church confess.
The way in which God answers your question is in the mystery of the virgin birth. The Scriptures begin from, what one theologian terms, “above” (Donald Macleod). John 1 or Philippians 2 show this movement from the eternal Son of God to the incarnate Son of God. This is how the Westminster Shorter Catechism summarizes it:
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)
As Matthew described it, Mary was found to be with child from the Holy Spirit (1:18), and then the angel explained it to Joseph: “that which is conceived in her [Mary] is from the Holy Spirit” (1:20). The explanation to Mary herself, in response to her “How will this be, since I am a virgin?” (Luke 1:34), leaves any human agency out of the incarnation in the normal way of conception, for the angel tells Mary, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore, the child to be born will be called ‘holy’—the Son of God” (Luke 1:35). Without further description of how this would occur, the Bible testifies that the Holy Spirit overshadowed Mary (the same idea as the cloud which overshadowed the Mount of Transfiguration) in such a way that she indeed became the bearer of the Holy One of God (Gal. 4:4); and her offspring, Jesus Christ, remained without sin. God does not specify in greater detail than this how the conception of Jesus by-passed Mary’s own sinfulness to preserve Christ’s sinlessness, but the rest of Scripture, as I indicated above, affirms that Christ did not inherit our sin nature. He came to be the Second Adam (Rom. 5) who could forgive sins because he himself was both the One offended and himself sinless.
The glory of the grace found in Jesus Christ is that though he was without sin, yet he had pity upon sinners so as to die in their place while we were enemies, ungodly, and sinful (Rom. 5:6, 8, 10). We need a Savior who is fully human to bear patiently with us, yet is able to atone for our sin as the final and perfect sacrifice. The sinlessness of Christ makes a passage like Isaiah 53:9–11 so marvelous in that Christ had no deceit in his mouth and by his death he made many to be accounted righteous.
I hope this is of some help.
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