The Catechism topic for today is faith in God the Father, God the Son, and God the Holy Spirit. There is some supporting information from the Summa Theologica as well.

II. "I Know Whom I Have Believed"16

To believe in God alone

150 Faith is first of all a personal adherence of man to God. At the same time, and inseparably, it is a free assent to the whole truth that God has revealed. As personal adherence to God and assent to his truth, Christian faith differs from our faith in any human person. It is right and just to entrust oneself wholly to God and to believe absolutely what he says. It would be futile and false to place such faith in a creature.17

To believe in Jesus Christ, the Son of God

151 For a Christian, believing in God cannot be separated from believing in the One he sent, his "beloved Son", in whom the Father is "well pleased"; God tells us to listen to him.18 The Lord himself said to his disciples: "Believe in God, believe also in me."19 We can believe in Jesus Christ because he is himself God, the Word made flesh: "No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known."20 Because he "has seen the Father", Jesus Christ is the only one who knows him and can reveal him.21

To believe in the Holy Spirit

152 One cannot believe in Jesus Christ without sharing in his Spirit. It is the Holy Spirit who reveals to men who Jesus is. For "no one can say "Jesus is Lord", except by the Holy Spirit",22 who "searches everything, even the depths of God. . No one comprehends the thoughts of God, except the Spirit of God."23 Only God knows God completely: we believe in the Holy Spirit because he is God.

The Church never ceases to proclaim her faith in one only God: Father, Son and Holy Spirit.

IN BRIEF

178 We must believe in no one but God: the Father, the Son and the Holy Spirit.

St. Paul writes of his belief in the Father, Son, and Holy Spirit in 2 Tim 1:12-14.

On this account I am suffering these things; but I am not ashamed, for I know him in whom I have believed and am confident that he is able to guard what has been entrusted to me until that day. Take as your norm the sound words that you heard from me, in the faith and love that are in Christ Jesus. Guard this rich trust with the help of the holy Spirit that dwells within us.

St. Paul writes of his belief in the Father, Son, and Holy Spirit in 2 Tim 1:12-14.

On this account I am suffering these things; but I am not ashamed, for I know him in whom I have believed and am confident that he is able to guard what has been entrusted to me until that day. Take as your norm the sound words that you heard from me, in the faith and love that are in Christ Jesus. Guard this rich trust with the help of the holy Spirit that dwells within us.

In the Summa Theologica by St. Thomas Aquinas (2, 2, 2), the great doctor of the Church explains that it is necessary to believe in the Trinity:

Article 8. Whether it is necessary for salvation to believe explicitly in the Trinity?

Objection 1. It would seem that it was not necessary for salvation to believe explicitly in the Trinity. For the Apostle says (Hebrews 11:6): "He that cometh to God must believe that He is, and is a rewarder to them that seek Him." Now one can believe this without believing in the Trinity. Therefore it was not necessary to believe explicitly in the Trinity.

Objection 2. Further our Lord said (John 17:5-6): "Father, I have manifested Thy name to men," which words Augustine expounds (Tract. cvi) as follows: "Not the name by which Thou art called God, but the name whereby Thou art called My Father," and further on he adds: "In that He made this world, God is known to all nations; in that He is not to be worshipped together with false gods, 'God is known in Judea'; but, in that He is the Father of this Christ, through Whom He takes away the sin of the world, He now makes known to men this name of His, which hitherto they knew not." Therefore before the coming of Christ it was not known that Paternity and Filiation were in the Godhead: and so the Trinity was not believed explicitly.

Objection 3. Further, that which we are bound to believe explicitly of God is the object of heavenly happiness. Now the object of heavenly happiness is the sovereign good, which can be understood to be in God, without any distinction of Persons. Therefore it was not necessary to believe explicitly in the Trinity.

On the contrary, In the Old Testament the Trinity of Persons is expressed in many ways; thus at the very outset of Genesis it is written in manifestation of the Trinity: "Let us make man to Our image and likeness" (Genesis 1:26). Therefore from the very beginning it was necessary for salvation to believe in the Trinity.

I answer that, It is impossible to believe explicitly in the mystery of Christ, without faith in the Trinity, since the mystery of Christ includes that the Son of God took flesh; that He renewed the world through the grace of the Holy Ghost; and again, that He was conceived by the Holy Ghost. Wherefore just as, before Christ, the mystery of Christ was believed explicitly by the learned, but implicitly and under a veil, so to speak, by the simple, so too was it with the mystery of the Trinity. And consequently, when once grace had been revealed, all were bound to explicit faith in the mystery of the Trinity: and all who are born again in Christ, have this bestowed on them by the invocation of the Trinity, according to Matthew 28:19: "Going therefore teach ye all nations, baptizing them in the name of the Father, and of the Son and of the Holy Ghost."

Reply to Objection 1. Explicit faith in those two things was necessary at all times and for all people: but it was not sufficient at all times and for all people.

Reply to Objection 2. Before Christ's coming, faith in the Trinity lay hidden in the faith of the learned, but through Christ and the apostles it was shown to the world.

Reply to Objection 3. God's sovereign goodness as we understand it now through its effects, can be understood without the Trinity of Persons: but as understood in itself, and as seen by the Blessed, it cannot be understood without the Trinity of Persons. Moreover the mission of the Divine Persons brings us to heavenly happiness.

Footnotes

16 ⇒ 2 Tim 1:12
17 Cf. ⇒ Jer 17:5-6; ⇒ Pss 40:5; ⇒ 146:3-4
18 ⇒ Mk 1:11; cf. ⇒ 9:7
19 ⇒ Jn 14:1
20 ⇒ Jn 1:18.
21 ⇒ Jn 6:46; cf. ⇒ Mt 11:27
22 ⇒ I Cor 12:3
23 ⇒ I Cor 2:10-11.