For to invoke angels without having obtained a knowledge of their nature greater than is possessed by men, would be contrary to reason. But, conformably to our hypothesis, let this knowledge of them, which is something wonderful and mysterious, be obtained. Then this knowledge, making known to us their nature, and the offices to which they are severally appointed, will not permit us to pray with confidence to any other than to the Supreme God, who is sufficient for all things, and that through our Saviour the Son of God, who is the Word, and Wisdom, and Truth, and everything else which the writings of God's prophets and the apostles of Jesus entitle Him.
And it is enough to secure that the holy angels of God be propitious to us, and that they do all things on our behalf, that our disposition of mind towards God should imitate as far as it is within the power of human nature the example of these holy angels, who again follow the example of their God; and that the conceptions which we entertain of His Son, the Word, so far as attainable by us, should not be opposed to the clearer conceptions of Him which the holy angels possess, but should daily approach these in clearness and distinctness.
But because Celsus has not read our holy Scriptures, he gives himself an answer as if it came from us, saying that we “assert that the angels who come down from heaven to confer benefits on mankind are a different race from the gods,” and adds that “in all probability they would be called demons by us:” not observing that the name “demons” is not a term of indifferent meaning like that of “men,” among whom some are good and some bad, nor yet a term of excellence like that of “the gods,” which is applied not to wicked demons, or to statues, or to animals, but (by those who know divine things) to what is truly divine and blessed; whereas the term “demons” is always applied to those wicked powers, freed from the encumbrance of a grosser body, who lead men astray, and fill them with distractions and drag them down from God and supercelestial thoughts to things here below.
Source: Against Celsus (New Advent)