Luther: Christ "grants us His birth"

"The right and gracious faith which God demands is, that you firmly believe that Christ is born for you, and that this birth took place for your welfare. The Gospel teaches that Christ was born, and that he did and suffered everything in our behalf, as is here declared by the angel: “Behold, I bring you good tidings of great joy which shall be to all the people; for there is born to you this day a Savior, who is Christ the Lord.” In these words you clearly see that he is born for us.

"He does not simply say, Christ is born, but to you he is born, neither
does he say, I bring glad tidings, but to you I bring glad tidings of great
joy. Furthermore, this joy was not to remain in Christ, but it shall be to all
the people. This faith no condemned or wicked man has, nor can he have
it; for the right ground of salvation which unites Christ and the believing heart is that they have all things in common.

"...Christ’s birth can not be distributed in a material sense neither would that avail any thing; it is therefore imparted spiritually, through the Word, as the angel says, it is given to all who firmly believe so that no harm will come to them because of their impure birth. This it the way and manner in which we are to be cleansed from the miserable birth we have from Adam. For this purpose Christ willed to be born, that through him we might be born again, as he says in John 3:3, that it takes place through faith; as also St. James says in James 1:18: “Of his own will he brought us forth by the word of truth,
that we should be a kind of first-fruits of his creatures.”

"We see here how Christ, as it were, takes our birth from us and absorbs
it in his birth, and grants us his, that in it we might become pure and holy,
as if it were our own, so that every Christian may rejoice and glory in
Christ’s birth as much as if he had himself been born of Mary as was Christ...Christ must above all things become our own and we become his...

"Therefore see to it that you do not find pleasure in the Gospel only as a
history, for that is only transcient; neither regard it only as an example, for
it is of no value without faith; but see to it that you make this birth your own and that Christ be born in you." -- Martin Luther, Christmas Day sermon, from his Wartburg Church postil, 1521-22.