Taken from Notes Made at the Conferences of Dom Prosper Guéranger)
Genitum non factum, Begotten not made. We human creatures have all of us been made, we are the work of God, every one of us, not even excepting Our Blessed Lady and the Angels. But as to the Word, the Son of God, it is not so: He is Begotten, not made; He came forth from the Father, but He is not His work. He has the same Substance, the same Essence, the same Nature as the Father. In God, it behooves us ever to make distinction of Persons, but we must also ever behold the same Divine Substance, as well for the Father and the Son, as for the Holy Ghost: idem quoad substantiam. Our Lord also tells us so Himself: Ego et Pater unum sumus; they are One, but the Persons are distinct; Father, Son, and Holy Ghost, these are the three terms which serve to designate them. Very important indeed, then, is this great word of the Council of Nicaea: Consubstantialem Patri, consubstantial with the Father. Yea, the Son is Begotten by the Father, He has the same Substance; there is the same Divine Essence.
Per quem omnia facta sunt, by Whom all things were made. It was said at the beginning of the Symbol, that God made Heaven and earth and all creatures visible and invisible; and now here we are told, speaking of the Word, the Son of God, that all things were made by Him. How are we to reconcile all this? It can easily be understood by means of a comparison with our own soul. Three distinct faculties are given her, for the exercising of these her three distinct acts: power, understanding, and will. These three faculties are necessary to the perfecting of an act. By power, the soul is enabled to act, but this presupposes understanding and will. In like manner God the Almighty Father has made all things by His Power; he has made all things in Wisdom by His Son; and thereon has stamped His Will by the Holy Ghost: and thus is His Act perfected. It is therefore quite correct to say, speaking of the Son: per quem omnia facta sunt.