4 But why Matthew begins with Abraham and descends to Joseph, while Luke begins with Joseph and ascends, not to Abraham, but to God, who made man, and, by giving a commandment, gave him power to become, by believing, a son of God; and why Matthew records the generations at the commencement of his book, Luke after the baptism of the Saviour by John; and what is the meaning of the number of the generations in Matthew, who divides them into three sections of fourteen each, though in the whole sum there appears to be one wanting; while in Luke the number of generations recorded after the baptism amount to seventy-seven, which number the Lord Himself enjoins in connection with the forgiveness of sins, saying, "Not only seven times, but seventy-seven times;"— these things you will never understand, unless either you are taught by some Catholic of superior stamp, who has studied the sacred Scriptures, and has made all the progress possible, or you yourselves turn from your error, and in a Christian spirit ask that you may receive, seek that you may find, and knock that it may be opened to you.
Source: Reply to Faustus the Manichaean (New Advent)