The Catechism in Questions & Answers

28. What is the foundation and content of Christian faith?

Christians believe in the one God— Father, Son, and Holy Spirit. Belief in the triune God has been made accessible to mankind by Jesus Christ.
The Son, Jesus Christ, spoke of His heavenly Father, in whom human beings are to believe. On several occasions,
God, the Father, attested that Jesus Christ is His Son (cf. Luke 3: 22; 9: 35).
Finally, Jesus Christ promised that the Holy Spirit would come as a Comforter and Helper.

Concerning the fundamentals of the Christian faith: see also Questions 34. and 35.

“Baptising them in the name of the Father and of the Son and of the Holy Spirit.”
Matthew 28: 19
“But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all things that I said to you.”
John 14: 26