Rev. 3.10 says if "thou didst keep the word of my patience, I also will keep thee from the hour of trial, that hour which is to come upon the whole world, to try them that dwell upon the earth." The only way for God to keep this promise is through rapture. There is no guarantee a Christian won't be martyrded during the Tribulation.

Therefore, only Christians who do this will be raptured before the Tribulation. Those who are alive and left will be raptured at the start of the last trumpet (1 Cor. 15.50-52, 1 Thess. 4.14-18).