Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 25

Chapter 25.—Concerning Those Things Which May Be Common to the Holy Angels and to Men.

Wherefore we must by no means seek, through the supposed mediation of demons, to avail ourselves of the benevolence or beneficence of the gods, or rather of the good angels, but through resembling them in the possession of a good will, through which we are with them, and live with them, and worship with them the same God, although we cannot see them with the eyes of our flesh.  But it is not in locality we are distant from them, but in merit of life, caused by our miserable unlikeness to them in will, and by the weakness of our character; for the mere fact of our dwelling on earth under the conditions of life in the flesh does not prevent our fellowship with them.  It is only prevented when we, in the impurity of our hearts, mind earthly things.  But in this present time, while we are being healed that we may eventually be as they are, we are brought near to them by faith, if by their assistance we believe that He who is their blessedness is also ours.