NOTES for Mat 1:20-21
Each evangelist describes the Nativity, setting the emphases in his own way. Matthew pays special attention to Joseph. It would seem that in this case Joseph's role is more that of an observer than of an active participant. But that is only at first glance. And it is not only that he had to accept what was happening as part of his own life. The point is that on earth he was to become, in effect, the adoptive father of the Messiah. He did not become, and could not become, His father by blood. But every child, even one born as the Messiah was born, needs a father. Not only the heavenly Father, but also an earthly father. If not a biological one, then an adoptive one. After all, the Messiah had to grow up as a Man. It is not easy for a boy without a father to grow up as a man with a capital M, even when we understand that capital letter in human terms.
If the Messiah had been meant to grow up less a Man than anyone else, perhaps He would not have needed an earthly father at all. But He had to grow up more truly Man than any of us. And therefore an earthly father was absolutely necessary for Him. Only such a father had to be one who could in no way and by nothing harm Him, who could not even inadvertently introduce into His human nature anything incompatible with the fullness of God that the Messiah bears within Himself from the very moment of His conception.
There is little hope in flesh and blood here, not because they are nature, but because in humanity's present condition they are fallen nature. And until the complete transfiguration of the human person, this cannot be corrected. The spiritual life of a person is different: here, even in the fallen state, much can be achieved. A person's soul is more easily corrected than his body. And if it does not become ideal, it is at least possible to make sure that its sinfulness does not poison the life of one's neighbor.
This is, of course, not simple. Direct help from God and effort from the person himself are needed, but a result can be achieved here. Therefore the adoptive father remained the only option in the case of the Messiah, whose human nature had to remain free from sin. And Joseph's consent to this path was required. By taking his wife, he gives God that consent. And he becomes the earthly father of the Messiah. Adoptive, but no less real for that.