The Catechism in Questions & Answers

574. What events comprise the first resurrection?

During the first resurrection, the “dead in Christ” will resurrect and, together with the living who belong to the bridal congregation, will be caught up to God. After the “marriage in heaven”, the martyrs from the “great tribulation” will resurrect and be numbered among the royal priesthood.
These two events are described as the “first resurrection”: “Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years” (Revelation 20: 6).

Rapture / transformation: see Question 559. et seq. “Great tribulation”: see Questions 569. et seq.