The Catechism in Questions & Answers

458. Why is the Chief Apostle ministry also called the “ministry of the rock”?

Jesus described Apostle Simon as the “rock” upon which He would build His church. Thereby the Son of God created an indissoluble relationship between the ministry of Peter—the ministry of the rock—and the church of Christ. Today the “ministry of the rock” is discharged by the Chief Apostle.

“Peter” is the modern form of the Greek word petros, which means “rock”. The rock is an image of strength, changelessness, and steadfastness, which the Lord Jesus also used in the Sermon on the Mount (cf. Matthew 7: 24-25).