Therefore, if one of the members of the Trinity was going to become a substitutionary sacrifice that was going to cancel out our sin debt, then one of the persons in the Trinity had to become embodied, that is to take on human form. This was because God had decreed that blood is to be used for the atonement of sins. It tells us this a great many times all through both the Old and New Testaments.
In the Old testament verses such as Leviticus 17:11 where God says, "For the life of the flesh is in the blood, and I have given it to you upon the altar to make atonement for your sins; for it is the blood that makes atonement for the sins of the soul."
In the New Testament verses like 1 John 1:7, "And the blood of Jesus his Son cleanses us from all sin".
The problem was that if Christ were to come into the world through a natural human pregnancy, then the seed of the sinful man, when deposited into the womb of a woman, would cause the blood of the newborn Jesus to be tainted blood. In other words the sinful nature of the human father would then be passed right along to the newborn Christ. Thus God the Father bypassed the human father and instead planned and prepared the body of Jesus Christ. Hebrews 10:5 has Jesus telling us that his Father indeed prepared his earthly body, "But a body You have prepared for Me."
So you can see that since animal blood was inadequate and man's blood was tainted, the only solution was that a member of the Godhead had to take the form of a human body containing the pure sinless blood that was from God the Father.
Adam and Jesus were the only human beings who ever had their blood given to them directly from God himself. (Eve had been given the blood from Adam when she was made from his rib.) Unfortunately the first Adam allowed sin to enter into this world and infect all the humans that followed. Jesus, the last Adam, made it possible for all humans to be washed clean of all their sins.
And this sinless seed from God the Father was miraculously placed in the womb of the virgin by the Holy Spirit, and this resulted in the incarnation - God in the flesh, with sinless blood. Jesus Christ became the begotten son of God the Father. (Begotten simply means born of flesh).
That is why 1 Peter 1:19 declares that we are redeemed with the precious
blood of Christ, like a lamb without blemish and without spot."
Christ's blood is precious because it was never tainted with the sinful human nature of man. So man's redemption and salvation stands or falls upon the foundation of this virgin birth.
The blessed, all-powerful Holy Spirit was the agent used in carrying out this miracle of the ages. The story unfolds in Luke chapter one. The angel appears to Mary and says: . . . thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. Mary, in her her astonishment replies, "How shall this be, seeing I know not a man?" The word "know" in Scripture often refers to the sex act. For instance, Genesis 4:1 says: And Adam knew Eve his wife; and she conceived . . . . So Mary is actually saying, "How shall I have a child when I have never known a man intimately?"
The angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Highest shall overshadow thee. Therefore also that holy thing which shall be born of thee shall be called the Son of God . Blessed be the wonder-working, omnipotent Holy Spirit who took the body and blood prepared by the Father and placed the Christ - the sent One - into the womb of the pure virgin.