We have arrived, as the Lord has willed it, to the day of our promise. He will grant this also, that we may arrive at the fulfillment of the promise. For then those things which we say, if they are useful to us and to you, are from Him; but those things which proceed from man are false, as our Lord Jesus Christ Himself has said, “He that speaks a lie speaks of his own.” No one has anything of his own except falsehood and sin. But if man has any truth and justice, it is from that fountain after which we ought to thirst in this desert, so that being, as it were, bedewed by some drops from it, and comforted in the meantime in this pilgrimage, we may not fail by the way, but reach His rest and satisfying fullness.
If then “he that speaks a lie speaks of his own,” he who speaks the truth speaks of God. John is true, Christ is the Truth; John is true, but every true man is true from the Truth. If, then, John is true, and a man cannot be true except from the Truth, from whom was he true, unless from Him who said, “I am the truth”? The Truth, then, could not speak contrary to the true man, or the true man contrary to the Truth. The Truth sent the true man, and he was true because sent by the Truth.
If it was the Truth that sent John, then it was Christ that sent him. But that which Christ does with the Father, the Father does; and what the Father does with Christ, Christ does. The Father does nothing apart from the Son, nor the Son anything apart from the Father: inseparable love, inseparable unity: inseparable majesty, inseparable power, according to these words which He Himself propounded, “I and my Father are one.” Who then sent John? If we say the Father, we speak truly; if we say the Son, we speak truly; but to speak more plainly, we say the Father and the Son.
But whom the Father and the Son sent, one God sent; because the Son said, “I and the Father are one.” How, then, did he not know Him by whom he was sent? For he said, “I knew Him not: but He that sent me to baptize with water, the same said unto me.” I interrogate John: “Who sent you to baptize with water? What did He say to you?” “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He which baptizes with the Holy Ghost.” Is it this, O John, that He said to you who sent you?
It is manifest that it was this; who, then, sent you? Perhaps the Father. True God is the Father, and the Truth is God the Son: if the Father without the Son sent you, God without the Truth sent you; but if you are true, because you speak the truth, and dost speak of the Truth, the Father did not send you without the Son, but the Father and the Son together sent you. If, then, the Son sent you with the Father, how did you not know Him by whom you were sent? He whom you had seen in the Truth, Himself sent you that He might be recognized in the flesh, and said, “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He which baptizes with the Holy Ghost.”
Source: Tractates on the Gospel of John (New Advent)