Christ also conversed in person with the apostles, and He sent the grace of the Spirit to be their teacher. But later, heresies would arise and our morals would be corrupted. Therefore it was His good pleasure that the Gospels be written down in order to teach us the truth, so that we would not be drawn away by the falsehood of these heresies, and our morals altogether corrupted. He gave us four Gospels, perhaps because we learn from them the four universal virtues: courage, prudence, righteousness, and self-control. We learn courage when the Lord says, Fear not them which kill the body, but are not able to kill the soul (Mt 10:28); we learn prudence when He exhorts, Be ye wise therefore as serpents (Mt 10:16); we learn righteousness when He teaches, Therefore all things whatsoever ye would that men should do to you, do ye even so to them (Mt 7:12); and we learn self-control when He declares, Whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart (Mt 5:28).
For another reason are there four Gospels: they are pillars of the world. As the world is divided into four parts—east, west, north, and south—it was right that there also be four pillars. And for yet another reason are the Gospels four in number: they contain four elements—teachings, commandments, warnings, and promises. To those who believe the teachings and observe the commandments, God promises the good things that are to come. But those who do not believe the teachings and do not keep the commandments, He threatens with the punishments that are to come.