Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 9

Chapter 9.—That All the Miracles Which are Done by Means of the Martyrs in the Name of Christ Testify to that Faith Which the Martyrs Had in Christ.

To what do these miracles witness, but to this faith which preaches Christ risen in the flesh, and ascended with the same into heaven?  For the martyrs themselves were martyrs, that is to say, witnesses of this faith, drawing upon themselves by their testimony the hatred of the world, and conquering the world not by resisting it, but by dying.  For this faith they died, and can now ask these benefits from the Lord in whose name they were slain.  For this faith their marvellous constancy was exercised, so that in these miracles great power was manifested as the result.  For if the resurrection of the flesh to eternal life had not taken place in Christ, and were not to be accomplished in His people, as predicted by Christ, or by the prophets who foretold that Christ was to come, why do the martyrs who were slain for this faith which proclaims the resurrection possess such power?  For whether God Himself wrought these miracles by that wonderful manner of working by which, though Himself eternal, He produces effects in time; or whether He wrought them by servants, and if so, whether He made use of the spirits of martyrs as He uses men who are still in the body, or effects all these marvels by means of angels, over whom He exerts an invisible, immutable, incorporeal sway, so that what is said to be done by the martyrs is done not by their operation, but only by their prayer and request; or whether, finally, some things are done in one way, others in another, and so that man cannot at all comprehend them,—nevertheless these miracles attest this faith which preaches the resurrection of the flesh to eternal life.