My personal opinion is that the great multitude taken to heaven are those killed by the Antichrist for not worshipping him & his image and any born again Christian remaining on earth who escaped his clutches who are all taken to heaven after the 6th seal is opened.  They are the ones who experience great tribulation and came out of great tribulation when they are taken to heaven because there is still great tribulation on Earth after they are taken to heaven.

I see the rapture/resurrection taking place when the Antichrist reveals himself as evidenced by those "accounted worthy" to escape all these things (Luke 21:36) and those kept from the hour of temptation that will come upon the whole world according to Rev. 3:10.

2 Thess. 2:1-4 says, "Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand. Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God."

So while we may see the Antichrist reveal himself when he revives from the deadly mortal wound to sit in the temple of God, I don't think we will be around on the Earth when the image of the beast is created because when the image of the beast known as the abomination of desolation is set up in the holy place, that is when the Antichrist has authority for 1,260 days to kill people, the woman (Israel) flees into the wilderness for 1,260 days, Jerusalem is trampled down by the Gentiles for 1,260 days, & the Two Witnesses prophesy for 1,260 days.  ( I have finally come to the conclusion that the Two Witnesses are here during the Great Tribulation from further Bible study and reading all the posts at 5doves about them.)

Also all the killing of the martyrs who will not worship the beast nor his image must take place before the sixth seal is opened as we see the great multitude up in heaven after the 6th seal is open.