The Lord Jesus Christ is the eternal Son of God. He has always been God, and He did not cease to be fully God at His incarnation, for “in him dwelleth all the fullness of the Godhead bodily” (Col 2:9). So when the Apostle Paul says that Christ “was made in the likeness of men” (Phil 2:7), he is really referring to Christ taking on human nature so that He is, from hence, fully God and fully man.
Christ must be fully man in order that He may be a true representative of man. But in order that He may save us by paying for the penalty due to our sin, and by representing us in righteousness, Christ must be sinlessly perfect. He must not only be without actual sin, but He must be without original sin, i.e., without guilt on account of Adam’s sin and without a corrupt nature. And not only that but if Christ were to fulfil the Old Testament prophecies concerning the coming Messiah (e.g., 2 Sam 7:13–14; Ps 132:11; Isa 9:6–7; 11:1–2), He must also be the true seed of David.
How could so many conditions be fulfilled in one man? With God nothing is impossible! First, in order that Christ be fully man, He had to be conceived in the womb of a woman like all men are. Secondly, in order that He be without original sin, He must be conceived in the womb of a virgin by the power of the Holy Spirit (cf. Lk 1:35; Isa 7:14). That way, unlike all men descending from Adam by natural generation, the Lord Christ would be without original guilt, seeing that He not be in the loins of Adam when he fell at his first transgression (cf. Heb 7:5, 10). Also, thirdly, by the overshadowing of the Holy Spirit, and the creation of a perfect human soul to be united with the body formed of the flesh and blood of the virgin, the Lord Christ was preserved from original corruption. Moreover, fourthly, that the prophecies concerning the seed of David be fulfilled, the virgin Mary, who was of the lineage of David (Lk 3:31), and was espoused to Joseph who was of the house of David (Mt 1:20; Lk 1:27), was chosen.
In the Gospel accounts, we are told that when Joseph found out that Mary was with child, he had assumed that she was unfaithful and was ready to put her away (Mt 1:19), but the angel of the Lord appeared unto him and assured him that the child was “conceived… of the Holy Ghost” (Mt 1:20). Joseph therefore took Mary to wife, but knew her not till she had delivered the Lord (Mt 1:24–25). In this way, the Lord was legally the son of David (by adoption), and naturally the son of David by lineage through Mary. So the Lord was a rightful heir to the throne of David.
The knowledge of this amazing arrangement that God has made for our salvation ought to fill our hearts with wonder and gratitude to the Lord for the love and care that He has shown us in providing for our redemption.