The Catechism in Questions & Answers

397. Did the church of Jesus Christ continue to grow?

Yes, even after the death of the early Apostles and despite the Christian persecutions, the church grew. People who believed in Jesus Christ and professed Him as their Lord received the sacrament of Holy Baptism with water, and so they were incorporated into the body of Christ. In this way the church of Jesus Christ spread throughout the whole world.

Body of Christ: see Questions 374. and 375.