5 But perhaps some one thinks that, as He Himself came not to us, but sent, we have not heard His own voice, but only the voice of those whom He sent. Far from it: let such a thought be banished from your hearts; for He Himself was in those whom He sent. Listen to Paul himself whom He sent; for Paul was specially sent as an apostle to the Gentiles; and it is Paul who, terrifying them not with himself but with Him says, “Do ye wish to receive a proof of Him who speaks in me, that is, of Christ?” Listen also to the Lord Himself. “And other sheep I have,” that is, among the Gentiles, “which are not of this fold,” that is, of the people of Israel: “them also must I bring.” Therefore, even when it is by the instrumentality of His servants, it is He and not another that brings them. Listen further: “They shall hear my voice.” See here also, it is He Himself who speaks by His servants, and it is His voice that is heard in those whom He sends. “That there may be one fold, and one shepherd.” Of these two flocks, as of two walls, is the corner-stone formed. And thus is He both door and the corner-stone: all by way of comparison, none of them literally.
Source: Tractates on the Gospel of John (New Advent)