3 When, therefore, we hear that the Son of God is the arm of God the Father, let no carnal custom raise its distracting din in our ears; but as far as His grace enables us, let us think of that power and wisdom of God by which all things were made. Surely such an arm as that is neither held out by stretching, nor drawn in by contracting it. For He is not one and the same with the Father, but He and the Father are one; and as equal with the Father, He is in all respects complete, as well as the Father: so that no room is left open for the abominable error of those who assert that the Father alone exists, but according to the difference of causes is Himself sometimes called the Son, sometimes the Holy Spirit; and so also from these words may venture to say, See you perceive that the Father alone exists, if the Son is His arm: for a man and his arm are not two persons, but one.
Not understanding nor considering how words are transferred from one thing to another, on account of some mutual likeness, even in our daily forms of speech about things the most familiar and visible; and how much the more must it be so, in order that things ineffable may find some sort of expression in our speech, things which, as they really exist, cannot be expressed in words at all? For even one man styles another his arm, by whom he is accustomed to transact his business: and if he is deprived of him, he says in his grief, I have lost my arm; and to him who has taken him away, he says, You have deprived me of my arm.
Let them understand, then, the sense in which the Son is termed the arm of the Father, as that by which the Father has executed all His works; that they may not, by failing to understand this, and continuing in the darkness of their error, resemble those Jews of whom it was said, “And to whom has the arm of the Lord been revealed?”
Source: Tractates on the Gospel of John (New Advent)