But being proved wrong so far, men betake themselves to saying, that he who sends is greater than he who is sent: therefore the Father is greater than the Son, because the Son continually speaks of Himself as being sent by the Father; and the Father is also greater than the Holy Spirit, because Jesus has said of the Spirit, Whom the Father will send in my name; and the Holy Spirit is less than both, because both the Father sends Him, as we have said, and the Son, when He says, But if I depart, I will send Him unto you. (St. Augustine, On the Trinity, 2.5.7)
By this point in the treatise, Augustine has demonstrated the unity of the Divine nature and will, the trinity of Persons, the full divinity of each, the order of processions, the one inseparable order of working, the double account of the savior (the “canonical rule”), and is in process of answering objections to the full co-equality of the Persons.
In our day, the above objection is not so much cast in the language of greater/lesser, but in the language of authority/submission. That the Son is sent by the Father “proves” that the Son was in subordination to the Father in eternity (“functionally” or otherwise). On the face of it, the claim looks quite reasonable and even Biblical. Clearly Christ says He is sent of the Father, and each together send the Spirit. And we are familiar with passages such as the following: