[B]ecause he had enjoined on them great things, to raise their courage He reassures them that He will be with then always, “even to the end of the world.” Now do you see the relation of His glory to His previous condescension? His own proper power is again restored. What He had said previously was spoken during the time of His humiliation. He promised to be not only with these disciples but also with all who would subsequently believe after them. Jesus speaks to all believers as if to one body. Do not speak to me, He says, of the difficulties you will face, for “I am with you,” as the one who makes all things easy. Remember that this is also said repeatedly to the prophets in the Old Testament. Recall Jeremiah objecting that He is too young and Moses and Ezekiel shrinking from the prophet’s office. “I am with you” is spoken to all these people.
St. John Chrysostom (+407)
The Gospel of Matthew, Homily 90.2