21 Since then we have the prophecies, let faith abide with us. Let them fall who fall through unbelief, since they so will; but you have taken your stand on the rock of the faith in the Resurrection. Let no heretic ever persuade you to speak evil of the Resurrection. For to this day the Manichees say, that, the resurrection of the Saviour was phantom-wise, and not real, not heeding Paul who says, Who was made of the seed of David according to the flesh; and again, By the resurrection of Jesus Christ our Lord from the dead. And again he aims at them, and speaks thus, Say not in your heart, who shall ascend into heaven; or who shall descend into the deep?
That is, to bring up Christ from the dead; and in like manner warning as he has elsewhere written again, Remember Jesus Christ raised from the dead; and again, And if Christ be not risen, then is our preaching vain, and your faith also vain. Yea, and we are found false witnesses of God; because we testified of God that He raised up Christ, whom He raised not up. But in what follows he says, But now is Christ risen from the dead, the first fruits of them that are asleep; — And He was seen of Cephas, then of the twelve; (for if you believe not the one witness, you have twelve witnesses;) then He was seen of above five hundred brethren at once; (if they disbelieve the twelve, let them admit the five hundred;) after that He was seen of James, His own brother, and first Bishop of this diocese.
Seeing then that such a Bishop originally saw Christ Jesus when risen, do not thou, his disciple, disbelieve him. But you say that His brother James was a partial witness; afterwards He was seen also of me Paul, His enemy; and what testimony is doubted, when an enemy proclaims it? “I, who was before a persecutor, now preach the glad tidings of the Resurrection.”
Source: Catechetical Lectures (New Advent)