Important elements of Nicene Monarchism include the priority of the Father over the Son and the Holy Spirit, and the etiological principle that the Father is the cause/source of both the person and substance of the Son and Holy Spirit. Though post-Augustine Catholic theologians rarely place an emphasis on the above aspects of Trinitarian thought (unlike many Anglican and Eastern Orthodox theologians), I remained convinced that the Catholic tradition has never denied those teachings. For instance, I found vestiges of Nicene Monarchism in the thought of Thomas Aquinas, who taught:
One of the most important 'relational' distinctions between the three persons of the Trinity is that, "the Father is the principle of the whole Godhead" (P1.Q.39.A5), the "fontal principle of the entire divinity" (fontale principium totius divinitatis - Aquinas, Commentum in Lib. 1 Sententiarum, D.34.Q.2) [See THIS THREAD for more on this issue.]
In his commentary on the Gospel of John (verse 14:28), we read:
1971 One could also say, as Hilary does, that even according to the divine nature the Father is greater than the Son, yet the Son is not inferior to the Father, but equal. For the Father is not greater than the Son in power, eternity and greatness, but by the dignity of a grantor or source. For the Father receives nothing from another, but the Son, if I can put it this way, receives his nature from the Father by an eternal generation. So, the Father is greater because he gives; but the Son is not inferior, but equal, because he receives all that the Father has: "God has bestowed on him the name which is above every name" (Phil 2:9). For the one to whom a single act of existence (esse) is given, is not inferior to the giver. [LINK to online source.]
So, although I had found snippets of Nicene Monarchism in post-Augustine Catholic theologians, it was not until I had recently read the English translation of Matthias Joseph Scheeben's, Die Mysterien des Christentums (The Mysteries of Christianity), that I came across definitive support for Nicene Monarchism within the Catholic tradition. The following germane selections will be from the B. Herder Book Co. 1947 English edition, translated by Cyril Vollert.
From Chapter IV - The Productions of the Second and Third Persons, we read (all bold emphasis mine):
The term "generation" is of course employed, in the first place, to indicate that the production of the Second Person in God is wholly different from creation, the act by which non-divine beings come into existence. Creation is a free act of the divine will, whereby God calls into being things which of themselves were nothing, and communicates to them an existence which is essentially different from His own. But God brings forth His interior Word by communicating to Him His own being, His own substance. The Word proceeds from the Father's innermost substance, which passes over to the Word and places Him in full possession of the very nature that is proper to the Father. (Page 87.)
In God, in whom all that is found scattered in creatures is one, faith reveals to us the production of the Word from the substance of the Father. This Word is an intelligible image of its principle, because it proceeds from the latter's cognition and manifests it. It is likewise a real. substantial, personal image, because the cognition and also the object of the cognition, are expressed and impressed in this Word. The Second Person in the Godhead is produced because the First Person wills to utter and attest Himself, to express and manifest His nature. The Second Person receives the Father's nature in order to exhibit and manifest it in Himself. What then is to prevent us from saying that He is truly generated, nay, that in accord with the words of Holy Scripture, all fatherhood in heaven and on earth is so-called after the generating fatherhood of His principle? (Page 91.)
Then, in a footnote (#4, p. 91 ff.) Scheeben provides a quotation from Aquinas' Summa Contra Gentiles (IV.11), a work I had read in the 90's, long before my studies into Nicene Monarchism, and quite frankly, failed to recall its importance to Nicene Monarchism. Note the following:
We must note that what is generated is said to be conceived, so long as it remains in the parent. God's Word is begotten of God in such wise that He does not depart from the Father but remains in Him. Therefore God's Word may rightly be said to be conceived of God. This is the reason why the Wisdom of God affirms: 'The depths were not as yet, and I was already conceived' (Prov. 8:24). (Page 92.)
A bit later, from the same footnote, we read:
Again, what is brought forth issues from the womb. For a similar reason the generation of God's Word, which is called birth to indicate his perfect distinction from His Father, is called birth from the womb, according to Psalm 109:3" 'From the womb before the day star I begot Thee." However, the distinction of the Word from the speaker does not prevent the Word from existing in the speaker. Hence, just as the Word is said t0 be begotten or brought forth from the womb, to indicate His distinct existence, so to show that this distinction does not exclude the Word form existence in the speaker, revelation assures us that He 'is in the bosom of the Father' (John 1:18).
Finally, we must advert to the fact that carnal generation of animals is effected by an active and a passive principle. The father has an active, the mother a passive part. Hence for procreation of offspring the father has one function, the mother a different one: the father confers nature and species on the progeny, whereas the mother, as passive and receptive principle, conceives and gives birth. Procession is predicated of the Word inasmuch as God understands Himself: but the divine intelligence involves no passive element, but is wholly active, so to speak, since the divine intellect is not in potency but exclusively in act. Therefore in the generation of God's Word there is no maternal function, but only a paternal function. Hence the various functions which pertain to the father and the mother in carnal generation, are attributed by Scripture to the Father in His generation of the Word: the Father is said to give life to the Son (cf. John 5:16), to conceive Him, and to bring Him forth. (Page 93.)
Towards the end of the chapter, Scheeben, provides some insightful commentary on the issue of 'relation' as it pertains to the three persons of the Godhead/Trinity. Scheeben writes:
The communication of the essence from one person to the others involves no separation or partition of the essence. On the contrary, the essence can be transmitted to one of the other persons only if this person enters into relationship with the First Person and is united to Him in oneness of essence.
Furthermore, the first principle is one, the original possessor of the divine nature is one, and the distinction among persons proceeds from this one principle. The distinction issues from the unity, and is in turn stabilized by this same unity. for the Second and Third Persons are distinct from the First Person only because they have their origin from Him and stand in relation to Him by virtue of this origin...(Page 115.)
The Father unites the other two persons with and in Himself as their common root and source; for He is the common principle of both. (Page 116.)
In ending, I would like to say that it is quite reassuring (and refreshing) to discover a 'heavy-weight' Catholic theologian who espouses a number of the propositions concerning the Godhead that I have been defending over the last few years.
Grace and peace,