This He denied not, when it was said to Him that the Father had commanded His Angels that they should minister unto Him and should serve Him.

Wherefore, it is evident to us that these creatures are in a very great number; since His Spouse and Secretary, Holy Church, of whom Solomon says: "Who is this that cometh forth from the Desert, full of those things which give delight, leaning upon her friend ?" says, believes, and preaches these most noble creatures to be almost innumerable; and She divides them into three Hierarchies, that is to say, three holy, or rather Divine, Principalities: and each Hierarchy has three orders, so that nine orders of spiritual creatures the Church holds and affirms.
The first is that of the Angels, the second of the Archangels, the third of the Thrones; and these three orders make the first Hierarchy--not first as to nobility, nor as to creation, for the others are more noble, and all were created together, but first in degree, according to our perception of their exaltation.
Then there are the Dominations; after them the Virtues; then the Principalities; and these make the second Hierarchy.
Above these are the Powers and the Cherubim, and above all are the Seraphim; and these make the third Hierarchy.
And the most potent reason for their contemplation is the number in which the Hierarchies are, and that in which the orders are.

For, since the Divine Majesty is in Three Persons, which have one substance, it is possible to contemplate them triply.

For it is possible to contemplate the Supreme Power of the Father, which the first Hierarchy gazes upon, namely, that which is first by nobility, and which we enumerate last.

And it is possible to contemplate the Supreme Wisdom of the Son; and upon this the second Hierarchy gazes.
And it is possible to contemplate the Supreme and most fervent Charity of the Holy Spirit; and upon this the third Hierarchy gazes, which, being nearest to us, gives of the gifts which it receives.
And, since it is possible to regard each person in the Divine Trinity triply, so in each Hierarchy there are three orders which contemplate diversely.