In order for Jesus to be Jesus (meaning “Yahweh is salvation”) He has to be Immanuel (meaning “God with us”).
When the angel commands Joseph to name the child “Jesus”, he also gives him the reason why, “for He will save His people from their sins.” This is an allusion to Psalm 130:8. In this Psalm the “He” who redeems Israel from his iniquities is Yahweh. Only God can forgive sins ultimately. It is His prerogative; He is the most offended party (Psalm 51:4). You do not have the right to forgive a debt against someone else. The scribes theology was sound when they questioned, “Who can forgive sins but God?” (Mark 2:1-11).
In the 11th century Anslem of Canterbury wrote an important book titled, Cur Deus Homo, loosely translated, Why the God-Man? Why did Jesus have to be Immanuel to deal with sins? Why must the second person of the Trinity take on human flesh? His answer, in short, is that in sin we incur a debt that only man ought to pay and only God can pay. Thus, in order to pay this debt, a God-Man is needed.
In creation, the law, and our conscience we know God above us and against us. Only in the gospel do we know God for us and with us—God incarnate, born of a virgin, truly man, truly God—with us.
Christ, by highest Heav’n adored;
Christ the everlasting Lord;
Late in time, behold Him come,
Offspring of a virgin’s womb.
Veiled in flesh the Godhead see;
Hail th’incarnate Deity,
Pleased with us in flesh to dwell,
Jesus our Emmanuel.
– Hark! the Herald Angels Sing by Charles Wesley