1 Faustus said: Do I believe the gospel? Certainly. Do I therefore believe that Christ was born? Certainly not. It does not follow that because I believe the gospel, as I do, I must therefore believe that Christ was born. This I do not believe; because Christ does not say that He was born of men, and the gospel, both in name and in fact, begins with Christ's preaching. As for the genealogy, the author himself does not venture to call it the gospel. For what did he write?
"The book of the generation of Jesus Christ the Son of David." The book of the generation is not the book of the gospel. It is more like a birth-register, the star confirming the event. Mark, on the other hand, who recorded the preaching of the Son of God, without any genealogy, begins most suitably with the words, "The gospel of Jesus Christ the Son of God." It is plain that the genealogy is not the gospel. Matthew himself says, that after John was put in prison, Jesus began to preach the gospel of the kingdom; so that what is mentioned before this is the genealogy, and not the gospel.
Why did not Matthew begin with, "The gospel of Jesus Christ the Son of God," but because he thought it sinful to call the genealogy the gospel? Understand, then, what you have hitherto overlooked — the distinction between the genealogy and the gospel. Do I then admit the truth of the gospel? Yes; understanding by the gospel the preaching of Christ. I have plenty to say about the generations too, if you wish. But you seem to me now to wish to know not whether I accept the gospel, but whether I accept the generations.
Source: Reply to Faustus the Manichaean (New Advent)