6 After this, we make mention of heaven, and earth, and sea; of sun and moon; of stars and all the creation, rational and irrational, visible and invisible; of Angels, Archangels, Virtues, Dominions, Principalities, Powers, Thrones; of the Cherubim with many faces: in effect repeating that call of David's Magnify the Lord with me. We make mention also of the Seraphim, whom Esaias in the Holy Spirit saw standing around the throne of God, and with two of their wings veiling their face, and with two their feet, while with two they did fly, crying Holy, Holy, Holy, is the Lord of Sabaoth. For the reason of our reciting this confession of God, delivered down to us from the Seraphim, is this, that so we may be partakers with the hosts of the world above in their Hymn of praise.
Source: Catechetical Lectures (New Advent)