Since there is only one divine being, why do you speak of three: Father, Son, and Holy Spirit?
Deuteronomy 6:4 – Hear, O Israel: The Lord our God, the Lord is one.
1 Corinthians 8:4-6 – So then, about eating food sacrificed to idols: We know that “An idol is nothing at all in the world” and that “There is no God but one.” For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.
Matthew 3:16-17 – As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
Matthew 28:18-19 – Then Jesus came to them and said, “All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,
Luke 4:18 – “The Spirit of the Lord is on me, because he has anointed me to proclaim good news to the poor. He has sent me to proclaim freedom for the prisoners and recovery of sight for the blind, to set the oppressed free,
Isaiah 61:1 – The Spirit of the Sovereign Lord is on me, because the Lord has anointed me to proclaim good news to the poor. He has sent me to bind up the brokenhearted, to proclaim freedom for the captives and release from darkness for the prisoners,
John 14:26 – But the Advocate, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you.
John 15:26 – “When the Advocate comes, whom I will send to you from the Father—the Spirit of truth who goes out from the Father—he will testify about me.
2 Corinthians 13:14 – May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all.
Galatians 4:6 – Because you are his sons, God sent the Spirit of his Son into our hearts, the Spirit who calls out, “Abba, Father.”
Titus 3:5-6 – he saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit, whom he poured out on us generously through Jesus Christ our Savior,
Whoever wants to be saved should above all cling to the catholic faith.
Whoever does not guard it whole and inviolable will doubtless perish eternally.
Now this is the catholic faith: We worship one God in trinity and the Trinity in unity, neither confusing the persons nor dividing the divine being.
For the Father is one person, the Son is another, and the Spirit is still another.
But the deity of the Father, Son, and Holy Spirit is one, equal in glory, coeternal in majesty.
What the Father is, the Son is, and so is the Holy Spirit.
Uncreated is the Father; uncreated is the Son; uncreated is the Spirit.
The Father is infinite; the Son is infinite; the Holy Spirit is infinite.
Eternal is the Father; eternal is the Son; eternal is the Spirit: And yet there are not three eternal beings, but one who is eternal; as there are not three uncreated and unlimited beings, but one who is uncreated and unlimited.
Almighty is the Father; almighty is the Son; almighty is the Spirit: And yet there are not three almighty beings, but one who is almighty.
Thus the Father is God; the Son is God; the Holy Spirit is God: And yet there are not three gods, but one God.
Thus the Father is Lord; the Son is Lord; the Holy Spirit is Lord: And yet there are not three lords, but one Lord.
As Christian truth compels us to acknowledge each distinct person as God and Lord, so catholic religion forbids us to say that there are three gods or lords.
The Father was neither made nor created nor begotten; the Son was neither made nor created, but was alone begotten of the Father; the Spirit was neither made nor created, but is proceeding from the Father and the Son.
Thus there is one Father, not three fathers; one Son, not three sons; one Holy Spirit, not three spirits.
And in this Trinity, no one is before or after, greater or less than the other; but all three persons are in themselves, coeternal and coequal; and so we must worship the Trinity in unity and the one God in three persons.
Whoever wants to be saved should think thus about the Trinity.
It is necessary for eternal salvation that one also faithfully believe that our Lord Jesus Christ became flesh.
For this is the true faith that we believe and confess: That our Lord Jesus Christ, God’s Son, is both God and man.
He is God, begotten before all worlds from the being of the Father, and he is man, born in the world from the being of his mother — existing fully as God, and fully as man with a rational soul and a human body; equal to the Father in divinity, subordinate to the Father in humanity.
Although he is God and man, he is not divided, but is one Christ.
He is united because God has taken humanity into himself; he does not transform deity into humanity.
He is completely one in the unity of his person, without confusing his natures.
For as the rational soul and body are one person, so the one Christ is God and man.
He suffered death for our salvation. He descended into hell and rose again from the dead.
He ascended into heaven and is seated at the right hand of the Father.
He will come again to judge the living and the dead.
At his coming all people shall rise bodily to give an account of their own deeds.
Those who have done good will enter eternal life, those who have done evil will enter eternal fire.
This is the catholic faith.
One cannot be saved without believing this firmly and faithfully.