2 The evangelist thereafter brings forward what has formed the brief subject of today's reading, and says, “But though He had done so many miracles before them, yet they believed not on Him: that the saying of Isaiah the prophet might be fulfilled, which he spoke, Lord, who has believed our report and to whom has the arm of the Lord been revealed?” Where he makes it sufficiently plain that the Son of God is Himself the arm of the Lord; not that the person of God the Father is determined by the shape of human flesh, and that the Son is attached to Him as a member of His body; but because all things were made by Him, and therefore He is designated the arm of the Lord.
For as it is with your arm that you work, so the Word of God is styled His arm; because by the Word He elaborated the world. For why does a man, in order to do some work, stretch forth his arm, but because the doing of it does not straightway follow his word? And if he was endowed with such pre-eminent power that what he said was done without any movement of his body, then would his word be his arm. But the Lord Jesus, the only-begotten Son of God the Father, as He is no mere member of the Father's body, so is He no mere thinkable, and audible, and transitory word; for, as all things were made by Him, He was the word of God.
Source: Tractates on the Gospel of John (New Advent)