At the Rapture, all true believers will be taken to heaven. This is because we are each personally indwelt with the Holy Spirit, who is "the one who is holding [the antichrist] back", and God will not judge his own Spirit. This is unique among all the righteous of all ages. Only during this, the church age, are people indwelt in this way.
But after we're taken out, there will still be billions of people on earth. Those who are killed during this time will be those who become believers after the Rapture. Any and all who do this, defying the antichrist and refusing to take his mark, will be put to death (most likely by beheading). These are "they that came (lit. "are still coming") out of the Great Tribulation. But by God's mercy, there will be survivors at the end. These people will be the ones to repopulate the earth during the Millenium.
Remember that this time is also referred to as "the time of Jacob's (a.k.a. Israel) Trouble, meaning it concerns the nation of Israel and not the church. National Israel will be the focus during this time. So the Trib. isn't about the church at all, but about finishing the prophetic vision of Danel's "70 weeks".