5 And why can it have been, that when there were so many disciples, two write only from among the apostles, and two from among their followers? (For one that was a disciple of Paul, and another of Peter, together with Matthew and John, wrote the Gospels.) It was because they did nothing for vainglory, but all things for use.
“What then? Was not one evangelist sufficient to tell all?” One indeed was sufficient; but if there be four that write, not at the same times, nor in the same places, neither after having met together, and conversed one with another, and then they speak all things as it were out of one mouth, this becomes a very great demonstration of the truth.
6. “But the contrary,” it may be said, “has come to pass, for in many places they are convicted of discordance.” Nay, this very thing is a very great evidence of their truth. For if they had agreed in all things exactly even to time, and place, and to the very words, none of our enemies would have believed but that they had met together, and had written what they wrote by some human compact; because such entire agreement as this comes not of simplicity. But now even that discordance which seems to exist in little matters delivers them from all suspicion, and speaks clearly in behalf of the character of the writers.
But if there be anything touching times or places, which they have related differently, this nothing injures the truth of what they have said. And these things too, so far as God shall enable us, we will endeavor, as we proceed, to point out; requiring you, together with what we have mentioned, to observe, that in the chief heads, those which constitute our life and furnish out our doctrine, nowhere is any of them found to have disagreed, no not ever so little.
But what are these points? Such as follow: That God became man, that He wrought miracles, that He was crucified, that He was buried, that He rose again, that He ascended, that He will judge, that He has given commandments tending to salvation, that He has brought in a law not contrary to the Old Testament, that He is a Son, that He is only-begotten, that He is a true Son, that He is of the same substance with the Father, and as many things as are like these; for touching these we shall find that there is in them a full agreement.
And if among the miracles they have not all of them mentioned all, but one these, the other those, let not this trouble you. For if on the one hand one had spoken of all, the number of the rest would have been superfluous; and if again all had written fresh things, and different one from another, the proof of their agreement would not have been manifest. For this cause they have both treated of many in common, and each of them has also received and declared something of his own; that, on the one hand, he might not seem superfluous, and cast on the heap to no purpose; on the other, he might make our test of the truth of their affirmations perfect.
Source: Homilies on the Gospel of St. Matthew (New Advent)