By their nature, angels are active spirits endowed with reason, will and knowledge; they serve God, fulfill the will of His Providence and praise Him. They are incorporeal spirits, and because they belong to the invisible world, cannot be seen by our bodily eyes. St. John of Damascus writes: “When it is the will of God that angels should appear to those who are worthy, they do not appear as they are in their essence, but, transformed, take on such an appearance as to be visible to physical eyes.” In the book of Tobit, the angel accompanying Tobit and his son says of himself: “All these days I was visible to you, but I neither ate nor drank, this only appeared to your eyes” (Tobit 12:19).
But St. John of Damascus also writes: “An angel can only be called incorporeal and non-material in comparison with us. For in comparison with God, Who alone is beyond compare, everything seems coarse and material, only the divinity is totally non-material -and incorporeal.”