The Harmony Of The Gospels by St. Augustine
Chapter II.--An Explanation of the Sense in Which Christ is the Son of David, Although He Was Not Begotten in the Way of Ordinary Generation by Joseph the Son of David.
4. Thus, too, even if one were able to demonstrate that no descent, according to the laws of blood, could be claimed from David for Mary, we should have warrant enough to hold Christ to be the son of David, on the ground of that same mode of reckoning by which also Joseph is called His father. But seeing that the Apostle Paul unmistakably tells us that |Christ was of the seed of David according to the flesh,| how much more ought we to accept without any hesitation the position that Mary herself also was descended in some way, according to the laws of blood, from the lineage of David? Moreover, since this woman's connection with the priestly family also is a matter not left in absolute obscurity, inasmuch as Luke inserts the statement that Elisabeth, whom he records to be of the daughters of Aaron, was her cousin, we ought most firmly to hold by the fact that the flesh of Christ sprang from both lines; to wit, from the line of the kings, and from that of the priests, in the case of which persons there was also instituted a certain mystical unction which was symbolically expressive among this people of the Hebrews. In other words, there was a chrism; which term makes the import of the name of Christ patent, and presents it as something indicated so long time ago by an intimation so very intelligible.