There is therefore One whose favour we should seek, and to whom we ought to pray that He would be gracious to us— the Most High God, whose favour is gained by piety and the practice of every virtue. And if he would have us to seek the favour of others after the Most High God, let him consider that, as the motion of the shadow follows that of the body which casts it, so in like manner it follows, that when we have the favour of God, we have also the good-will of all angels and spirits who are friends of God.
For they know who are worthy of the divine approval, and they are not only well disposed to them, but they co-operate with them in their endeavours to please God: they seek His favour on their behalf; with their prayers they join their own prayers and intercessions for them. We may indeed boldly say, that men who aspire after better things have, when they pray to God, tens of thousands of sacred powers upon their side. These, even when not asked, pray with them, they bring succour to our mortal race, and if I may so say, take up arms alongside of it: for they see demons warring and fighting most keenly against the salvation of those who devote themselves to God, and despise the hostility of demons; they see them savage in their hatred of the man who refuses to serve them with the blood and fumes of sacrifices, but rather strives in every way, by word and deed, to be in peace and union with the Most High through Jesus, who put to flight multitudes of demons when He went about “healing,” and delivering “all who were oppressed by the devil.”
Source: Against Celsus (New Advent)