2 But, according to you, to believe the gospel is not only to obey its commands, but also to believe in all that is written in it; and, first of all, that God was born. But neither is believing the gospel only to believe that Jesus was born, but also to do what He commands. So, if you say that I do not believe the gospel because I disbelieve the incarnation, much more do you not believe because you disregard the commandments. At any rate, we are on a par till these questions are settled.
If your disregard of the precepts does not prevent you from professing faith in the gospel, why should my rejection of the genealogy prevent me? And if, as you say, to believe the gospel includes both faith in the genealogies and obedience to the precepts, why do you condemn me, since we both are imperfect? What one wants the other has. But if, as there can be no doubt, belief in the gospel consists solely in obedience to the commands of God, your sin is twofold. As the proverb says, the deserter accuses the soldier.
But suppose, since you will have it so, that there are these two parts of perfect faith, one consisting in word, or the confession that Christ was born, the other in deed or the observance of the precepts; it is plain that my part is hard and painful, yours light and easy. It is natural that the multitude should flock to you and away from me, for they know not that the kingdom of God is not in word, but in power. Why, then, do you blame me for taking the harder part, and leaving to you, as to a weak brother, the easy part? You have the idea that your part of faith, or confessing that Christ was born, has more power to save the soul than the other parts.
Source: Reply to Faustus the Manichaean (New Advent)