“It is said that after the death of the first three evangelists, the three Gospels were brought to John while he yet lived that he might see them and judge if they had been composed according to the truth. When John saw them he fully accepted the grace of the truth in them; and whatever the other evangelists had omitted, he himself completed, and whatever they had touched on briefly, he elaborated in his own Gospel. This was the beginning of theology.”

(St Theophylact, ‘Commentary on Matthew’)