From “The City of God”, Book 22:
But our martyrs are not our gods; for we know that the martyrs and we have both but one God, and that the same. Nor yet are the miracles which they maintain to have been done by means of their temples at all comparable to those which are done by the tombs of our martyrs. If they seem similar, their gods have been defeated by our martyrs as Pharaoh’s magi were by Moses. In reality, the demons wrought these marvels with the same impure pride with which they aspired to be the gods of the nations; but the martyrs do these wonders, or rather God does them while they pray and assist, in order that an impulse may be given to the faith by which we believe that they are not our gods, but have, together with ourselves, one God.
…but to our martyrs we build, not temples as if they were gods, but monuments as to dead men whose spirits live with God. Neither do we erect altars at these monuments that we may sacrifice to the martyrs, but to the one God of the martyrs and of ourselves; and in this sacrifice they are named in their own place and rank as men of God who conquered the world by confessing Him, but they are not invoked by the sacrificing priest. For it is to God, not to them, he sacrifices, though he sacrifices at their monument; for he is God’s priest, not theirs. The sacrifice itself, too, is the body of Christ, which is not offered to them, because they themselves are this body.