The Catechism in Questions & Answers

398. How did the Christian doctrine develop?

The expectation of the imminent return of Christ eventually faded into the background in the sermons. Belief in the life and activity of the Son of God, in His death and resurrection, was nevertheless kept alive.
Under the inspiration of the Holy Spirit, the early church creeds were enshrined in writing. At church gatherings known as “councils”, the doctrines of the Trinity of God and of the dual nature of Jesus Christ were formulated and proclaimed binding for Christian faith.

Trinity of God: see Questions 61. et seq. Early church creeds: see Questions 33. et seq. Council: see explanation of Question 33.