Let’s say a man gets saved after the rapture. For whatever reason, he accepts the mark of the beast. Will that man still be saved?
According to Rev. 14:9-12 everyone who worships the beast and his image or receives the mark of his name will be lost. Rev. 14:13 indicates that believers will be expected to die rather than do any of those things. This is one of the passages that tell us Eternal Security will not be given to tribulation believers. Rev. 16:15 is another.
David’s prayer in Pslam 51:11 that the Lord not cast him away or take the Holy Spirit from him (like He had with Saul in 1 Sam. 16:14) tells us no one from the Old Testament received eternal security either. It’s a blessing given only to the Church.