From the Council of Florence:
For when Latins and Greeks came together in this holy synod, they all strove that, among other things, the article about the procession of the holy Spirit should be discussed with the utmost care and assiduous investigation. Texts were produced from divine scriptures and many authorities of eastern and western holy doctors, some saying the holy Spirit proceeds from the Father and the Son, others saying the procession is from the Father through the Son. All were aiming at the same meaning in different words. The Greeks asserted that when they claim that the holy Spirit proceeds from the Father, they do not intend to exclude the Son; but because it seemed to them that the Latins assert that the holy Spirit proceeds from the Father and the Son as from two principles and two spirations, they refrained from saying that the holy Spirit proceeds from the Father and the Son. The Latins asserted that they say the holy Spirit proceeds from the Father and the Son
not with the intention of excluding the Father from being the source and principle of all deity, that is of the Son and of the holy Spirit,
nor to imply that the Son does not receive from the Father, because the holy Spirit proceeds from the Son, nor that they posit two principles or two spirations; but they assert that there is only one principle and a single spiration of the holy Spirit, as they have asserted hitherto. Since, then, one and the same meaning resulted from all this, they unanimously agreed and consented to the following holy and God-pleasing union, in the same sense and with one mind.
In the name of the holy Trinity, Father, Son and holy Spirit, we define, with the approval of this holy universal council of Florence, that the following truth of faith shall be believed and accepted by all Christians and thus shall all profess it: that the holy Spirit is eternally from the Father and the Son, and has his essence and his subsistent being from the Father together with the Son, and proceeds from both eternally as from one principle and a single spiration. We declare that when holy doctors and fathers say that the holy Spirit proceeds from the Father through the Son, this bears the sense that thereby also the Son should be signified,
according to the Greeks indeed as cause, and according to the Latins as principle of the subsistence of the holy Spirit, just like the Father.
And since the Father
gave to his only-begotten Son in begetting him everything the Father has, except to be the Father, so
the Son has eternally from the Father, by whom he was eternally begotten, this also, namely that
the holy Spirit proceeds from the Son.
ewtn.com/library/COUNCILS/FLORENCE.HTM
This is what we mean by “greater,” that the cause is greater ontologically and logically than the effect, so similarly the Father is “greater” than the Son (and the Spirit in the same way). We are saying that giving is greater than receiving, and Christ is receiving. Such wording is considered licit to both St. Gregory and St. John, and definitely more Fathers.
Not only does understanding Christ’s words that the Father is greater than he in this sense we’ve explained been used by later Fathers as well as earlier ones, but it also illuminates many of the Apostles’ expressions of the doctine of the Trinity, some of the early Church’s expressions of it, as well the Apostles’ Creed. After all, in the early Church and Scripture, the Father can be referred to as God in contrast to Christ, who is called Lord, and not called God in this instance. The Apostles’ Creed does a similar thing. We can take these passages as saying that Christ is not God, or, we can take these passages as saying that the Father is the “source or beginning of deity,” as the Council puts it, not contradicting that the Son is essentially Divine, just not the source of his Divine nature.
Christi pax.