It is not required for faith to assent to all idea of St. Thomas Aquinas. The Catholic Church has many theologians. The Catechism, however, does address the simplicity:
202 Jesus himself affirms that God is “the one Lord” whom you must love “with all your heart, and with all your soul, and with all your mind, and with all your strength”.6 At the same time Jesus gives us to understand that he himself is “the Lord”.7 To confess that Jesus is Lord is distinctive of Christian faith. This is not contrary to belief in the One God. Nor does believing in the Holy Spirit as “Lord and giver of life” introduce any division into the One God:
We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple.8
8 Lateran Council IV: DS 800
Denzinger, from Lateran Council IV:
Firmly we believe and we confess simply that the true God is one alone, eternal, immense, and unchangeable, incomprehensible, omnipotent and ineffable, Father and Son and Holy Spirit: indeed three Persons but one essence, substance, or nature entirely simple. The Father from no one, the Son from the Father only, and the Holy Spirit equally from both; without beginning, always, and without end; the Father generating, the Son being born, and the Holy Spirit proceeding; consubstantial and coequal and omnipotent and coeternal; one beginning of all, creator of all visible and invisible things, of the spiritual and of the corporal; who by His own omnipotent power at once from the beginning of time created each creature from nothing, spiritual, and corporal, namely, angelic and mundane, and finally the human, constituted as it were, alike of the spirit and the body. For the devil and other demons were created by God good in nature, but they themselves through themselves have become wicked. But man sinned at the suggestion of the devil. This Holy Trinity according to common essence undivided, and according to personal properties distinct, granted the doctrine of salvation to the human race, first through Moses and the holy prophets and his other servants according to the most methodical disposition of the time.
We, however, with the approval of the sacred Council, believe and confess with Peter Lombard that there exists a most excellent reality, incomprehensible indeed and ineffable, which truly is the Father, and the Son, and the Holy Spirit, at the same time three Persons, and anyone of the same individually; and so in God there is Trinity only, not a quaternity; because any one of the three Persons is that reality, namely, substance, essence or divine nature, which alone is the beginning of all things, beyond which nothing else can be found, and that reality is not generating, nor generated, nor proceeding, but it is the Father who generates, the Son who is generated, and the Holy Spirit who proceeds, so that distinctions are in Persons and unity in nature. Therefore, although “one is the Father, another the Son, and another the Holy Spirit, yet they are not different” * but what is the Father is the Son and the Holy Spirit entirely the same, so that according to the true and Catholic Faith they are believed to be consubstantial. For the Father from eternity by generating the Son gave His substance to Him according to which He Himself testifies: “That which the Father has given to me is greater than all things” [John 10:29]. But it cannot be said that He (the Father) has given a part of His substance to Him (the Son), and retained a part for Himself, since the substance of the Father is indivisible, namely, simple. But neither can it be said that the Father has transferred His substance to the Son in generating, as if He had given that to the Son which he did not retain for Himself; otherwise the substance would have ceased to exist. It is clear, therefore, that the Son in being born without any diminution received the substance of the Father, and thus the Father and the Son have the same substance, and so this same reality is the Father and the Son and also the Holy Spirit proceeding from both. But when Truth prays to the Father for His faithful saying: “I will that they may be one in us, as we also are one” John 17:22]: this word “one” indeed is accepted for the faithful in such a way that a union of charity in grace is understood, for the divine Persons in such a way that a unity of identity in nature is considered, as elsewhere Truth says: “Be . . . perfect, as also your heavenly Father is perfect” [Matt. 5:48 ], as if He said more clearly, “Be perfect” in the perfection of grace “as your heavenly Father is perfect” in the perfection of grace, that is, each in his own manner, because between the Creator and the creature so great a likeness cannot be noted without the necessity of noting a greater dissimilarity between them. If anyone, therefore, shall presume to defend or approve the opinion or doctrine of the above mentioned Joachim, let him be refuted as a heretic by all.