The Catechism in Questions & Answers

455. What is the objective of the activity of the Apostles?

The activity of the Apostles is aimed at building up the Lord’s work of redemption and leading it to its completion. This includes dispensing the sacraments in the manner desired by Jesus Christ. The Apostles see to it that the gospel is preached in unadulterated fashion and that the congregations have a divinely pleasing order.
Beyond that, the Apostles are to prepare the bridal congregation for the return of the Lord by preaching the gospel, proclaiming the forgiveness of sins, baptising with water and the Holy Spirit, and celebrating Holy Communion.

The Lord’s work of redemption: see explanation of Question 386.