"I believe in God": this first affirmation of the Apostles' Creed is also the most fundamental. The whole Creed speaks of God, and when it also speaks of man and of the world it does so in relation to God. The other articles of the Creed all depend on the first, just as the remaining Commandments make the first explicit. The other articles help us to know God better as he revealed himself progressively to men. "The faithful first profess their belief in God."
Source: Catechism of the Catholic Church (Vatican.va)