The Catechism in Questions & Answers

442. What expectation comprised the central point of apostolic proclamation already at that time?

The Apostles expected the return of Christ during their lifetimes and prepared the believers for it (cf. 1 Thessalonians 4: 14-18). This shows that it lies in the nature of the Apostle ministry to proclaim the return of Christ and to prepare the congregations for this event.