An angel is an intelligent essence, always in motion. It has free will, is incorporeal, serves God, and has been bestowed with immortality. Only the Creator understands its true nature.
John of DamascusNow all things have been filled with light, both heaven and earth and those beneath the earth; so let all creation sing Christ's rising, by which it is established.
John of DamascusHow can this come about?" Mary asked. "The Holy Spirit will come upon you," the angel answered, "and the power of the Most High will cover you with its shadow." And now you are the one who puts the question: "How can bread become Christ and wine His Blood?" I answer: "The power of the Holy Spirit will be at work to give us a marvel which surpasses understanding.
John of Damascus