The Works of Dionysius the Areopagite, tr. by John Parker, , at sacred-texts.com
What the traditional number of the Angels signifies.
This also is worthy, in my opinion, of intellectual attention, that the tradition of the Oracles concerning the Angels affirms that they are thousand thousands, and myriad myriads, accumulating and multiplying, to themselves, the supreme limits of our numbers, and, through these, shewing clearly, that the ranks of the Heavenly Beings cannot be numbered by us. For many are the blessed hosts of the supermundane minds, surpassing the weak and contracted measurement of our material number, and being definitely known by their own supermundane and heavenly intelligence and science alone, which is given to them in profusion by the supremely Divine and Omniscient Framer of Wisdom, and essentiating p. 54 Cause and connecting Force, and encompassing Term of all created things together.