That Jesus was Mary’s biological son is a significant biblical assertion, and yet Jesus was without sin. The context for what has become known as “the virgin conception” begins in Genesis 3:15. God is judging the serpent for deceiving Eve, and God announces to him that He would put enmity between the serpent and the woman and between his seed and her seed. It is notable that this serpent is identified later as Satan (Revelation 12:9; 20:2), and it is perhaps surprising that the serpent would have “seed.” Perhaps even more surprising, the woman also has “seed” (Genesis 3:15). God further predicted that the seed of the woman would crush the serpent’s head, while the serpent (not the serpent’s seed) would crush the heel of this seed of Eve. Taken at face value, the text points forward to a specific seed—a biological descendent of Eve, but not a seed of Adam—to crush the serpent. In the process, the seed of the woman would receive a wound from the serpent.
Next in this remarkable path toward Jesus being Mary’s biological son comes a prophecy in Isaiah that a virgin would be with child. While the child was still very young, God would give Judah victory over two oppressing kings (Isaiah 7:14–16). It is not clear in the immediate context whether that Hebrew term alma refers to a virgin or simply to a young woman, but it is clear that the prophecy was actually fulfilled by Jesus (Matthew 1:20–23). Jesus was indeed conceived in Mary (Matthew 1:20) and born of Mary (Matthew 1:21) while she was still a virgin (not simply a young woman). Matthew explicitly tells his readers that Joseph kept Mary a virgin until Jesus was born (Matthew 1:24–25). While we often refer to Jesus’ birth by Mary as the “virgin birth,” it is more accurate to understand that Jesus was both conceived in and born from Mary. She was His human mother, and Jesus was conceived in Mary by the Holy Spirit (Matthew 1:18, 20).
It is significant that Jesus is Mary’s biological son but not Joseph’s for several reasons:
First, as the biological son of only Mary, Jesus could fulfill the prophecy of Genesis 3:15—He was biologically of the seed of woman but not of the seed of man.
Second, Jesus could fulfill the prophecy of Isaiah 7:14–16—He was actually born of a woman who was still a virgin. While this is, of course, humanly impossible, it is possible with God (Luke 1:37). The angel explained to Joseph how this would take place (Matthew 1:20–25). Jesus would be conceived by the Holy Spirit, and that would be one of the reasons Jesus would be recognized as the Son of God (Luke 1:35).
Third, the fact that Jesus was not biologically descended from a man is important because Joseph was in the line of Coniah. God had declared that Coniah would not have a descendant rule on the throne of David (Jeremiah 22:24–30). While Jesus was legally Joseph’s son, He was not biologically descended from Joseph. Thus, Jesus was not from Coniah and could fulfill the Messianic prophecies without God breaking His word about Coniah’s descendants.
Fourth, it is significant that Jesus was not the biological son of Adam because of humanity’s sinful nature. Jesus was and is a man, but He was not biologically descended from a father related to Adam—Jesus came not from a man but from a woman, by the Holy Spirit. Thus, Jesus was not made in the image and likeness of Adam, as were Adam’s sons (see Genesis 5:3).
Adam had been given a direct command by God not to eat of the fruit of one particular tree (Genesis 2:16–17). When Adam and Eve disobeyed the command, Adam was held accountable for sin, resulting in the death of the human race (Romans 5:12, 17). Eve sinned as well, but she was not accountable in the same way, as she was deceived (1 Timothy 2:14). Adam was not deceived, as he had been given the instruction directly by God. God held Adam accountable for sin and the fall of mankind. It is significant, then, that Jesus was the seed of Eve and the biological son of Mary. He did not possess the image and likeness of Adam as did Adam’s other sons. He possessed the image of the invisible God (Colossians 1:15). All other men bore Adam’s image, which was stained with sin, but Jesus—by virtue of the conception by the Holy Spirit—did not. In fact, Paul contrasts Jesus with Adam in Romans 5, explaining that Adam brought death, but Jesus brought life (Romans 5:15–19).
While the Bible doesn’t provide the details of how the Holy Spirit conceived Jesus in Mary, it is clear that Jesus was born while Mary was still a virgin. That Jesus was Mary’s biological son and Joseph’s legal but not biological son is important and helps us understand how Jesus is able to be a sinless sacrifice for our sins. Jesus didn’t have a sin nature, and He never sinned, so He could pay for the sin of others. He alone was righteous. If He had been made in Adam’s likeness and image, Jesus would have had His own sin to deal with. But through the miracle of the virgin conception, Jesus remained qualified to be our Savior—the perfect, sinless, sacrifice for sin on our behalf.