What do Catholics believe?

"Our profession of faith begins with God, for God is the First and the Last, the beginning and the end of everything. The Credo begins with God the Father, for the Father is the first divine person of the Most Holy Trinity; our Creed begins with the creation of heaven and earth, for creation is the beginning and the foundation of all God's works."

---the Catechism of the Catholic Church, no. 198

Catholic belief is succinctly expressed in the profession of faith or credo called the Nicene Creed:

The Nicene Creed
I believe in one God,
the Father almighty,
maker of heaven and earth, of all things visible and invisible.

I believe in one Lord Jesus Christ, the Only Begotten Son of God, born of the Father before all ages. God from God, Light from Light, true God from true God, begotten, not made, consubstantial with the Father; through him all things were made. For us men and for our salvation he came down from heaven, and by the Holy Spirit was incarnate of the Virgin Mary, and became man. For our sake he was crucified under Pontius Pilate, he suffered death and was buried, and rose again on the third day in accordance with the Scriptures. He ascended into heaven and is seated at the right hand of the Father. He will come again in glory to judge the living and the dead and his kingdom will have no end.

I believe in the Holy Spirit, the Lord, the giver of life, who proceeds from the Father and the Son, who with the Father and the Son is adored and glorified, who has spoken through the prophets.

I believe in one, holy, catholic and apostolic Church. I confess one Baptism for the forgiveness of sins and I look forward to the resurrection of the dead and the life of the world to come. Amen.