Why were the Gospels of Mark and Luke included in the New Testament?

Answer 1

Irenaeus, bishop of Lyons, was a strong advocate for including the Gospels of Matthew, Mark, Luke and John in the New Testament. He said that just as there were four winds and four corners of the earth, so there should be four gospels.

Having had the opportunity to read the four gospels in parallel, the Church Fathers realised that there was a strong literary dependence among them, such that at least two must have been copied from the third. They judged, incorrectly as it happens, that Matthew's Gospel was the original and that Mark and Luke had been written as copies of Matthew. In fact, scholars now know that it was Mark that was the original, and that Matthew and Luke had been based on Mark.

Believing Mark and Luke to be derivatives of Matthew, the Church Fathers recognised value in them. They thought Mark, being shorter and without a nativity account, was a useful summary of Matthew. They thought of Lukeas being written for the gentiles, as well as valuing it for its emphasis on the poor. Finally, they felt that Acts of the Apostles, by the same author as Luke, must be included in the New Testament and that supported inclusion of Luke's Gospel.