2 Augustine replied: Well, in answer to your own questions, you tell us first that you believe the gospel, and next, that you do not believe in the birth of Christ; and your reason is, that the birth of Christ is not in the gospel. What, then, will you answer the apostle when he says, "Remember that Christ Jesus rose from the dead, of the seed of David, according to my gospel?" You surely are ignorant, or pretend to be ignorant, what the gospel is. You use the word, not as the apostle teaches, but as suits your own errors.
What the apostles call the gospel you depart from; for you do not believe that Christ was of the seed of David. This was Paul's gospel; and it was also the gospel of the other apostles, and of all faithful stewards of so great a mystery. For Paul says elsewhere, "Whether, therefore, I or they, so we preach, and so ye believed." They did not all write the gospel, but they all preached it. The name evangelist is properly given to the narrators of the birth, the actions, the words, the sufferings of our Lord Jesus Christ.
The word gospel means good news, and might be used of any good news, but is properly applied to the narrative of the Saviour. If, then, you teach something different, you must have departed from the gospel. Assuredly those babes whom you despise as semi-Christians will oppose you, when they hear their mother Charity declaring by the mouth of the apostle, "If any one preach another gospel than that which we have preached to you, let him be accursed." Since, then, Paul, according to his gospel, preached that Christ was of the seed of David, and you deny this and preach something else, may you be accursed! And what can you mean by saying that Christ never declares Himself to have been born of men, when on every occasion He calls Himself the Son of man?
Source: Reply to Faustus the Manichaean (New Advent)