I don't believe true Christians will be on earth during the tribulation except those chosen to stay who will need to lead non-believers to Christ. Those left behind will have many questions about what is happening on earth and they will need guidence from someone with knowledge of scripture. It will be their last chance for salvation...however it will cost them their life once they accept Christ. This is where the mark of the beast plays its role because most likely this mark will come into effect during the 7 year tribulation when the anti-Christ comes forth. Those not accepting the 666 mark in their forehead or right hand be slain...probably beheaded like John the Baptist.
The Mark Of The Beast ... accept the Mark of the Beast will experience the full measure of ... 16 that the mark is to be placed in the foreheads ... Where does Yahweh place his Seal & Signature ... atschool.eduweb.co.uk/sbs777/ prophecy/mob.html - 35k - Cached - More from this site
There will be a 7 year peace treaty with Isreal, however it will be broken in 3 1/2 years...then there will be torment on earth such as never before and the anti-Christ will show his true colors. The two witnesses, Enoch and Elijah (most likely) http://www.prophecyforum.com/witnesses.html, shall prophesy a thousand two hundred and threescore days...Revelation 11:3-13.
Some view Enoch as a likely candidate for one of the two witnesses in that he never died before he was translated to be with God. This would, of course, be in line with the general rule of Hebrews 9:27: ?And as it is appointed unto men once to die, but after this the judgment.? But we must remember that this is a general rule of Scripture and not necessarily binding to all of God?s Word. Many were resuscitated in the Old and New Testament only to die again (1 Kgs. 17:17-22; 2 Kgs. 4:32-35; 2 Kgs. 13:20, 21; Matt. 9:23-25; Luke 7:11-15; John 11:43, 44; Acts 9:36-40). In fact, there will be one generation of believers that will not experience death. They will be raptured out before the Antichrist arises and the tribulation hour begins (1 Thess. 4:16-17; 1 Cor. 15:51-52).
Elijah is the most likely of those so far as being one of the two witnesses. He was translated without dying (2 Kgs. 2:1-11), although we found that this wasn?t necessarily a criterion for being one of the two witnesses. He commanded the rain to be withheld for three and one-half years (1 Kgs. 17:1; Luke 4:25; James 5:17-18). He performed many miracles and was predicted to return before the day of the Lord to prepare the way for the Messiah?s coming (Mal. 3:1-3; 4:5-6).