Undoubtedly, the greatest revival in the history of mankind will be during the Tribulation. John tells us that in his vision he beheld “a great multitude, which no one could count, from every nation and all tribes and peoples and tongues, standing before the throne and before the Lamb, clothed in white robes” (Rev. 7:9). That passage goes on for a few more verses (verses 9-14), and it is clear that this great multitude are believers that were martyred during the Tribulation and are now rejoicing in victory in heaven and are worshipping God and Jesus the Lamb.
Many of them will no doubt come to salvation because of the shocking disappearances of the Rapture. Then, when the two witnesses and the 144,000 evangelists are put in place at the beginning of the Tribulation, many will be converted through their witness. Likewise, I imagine that many will also be saved by the witness of people all around them who heard the gospel message from others, even from angels. And of course, the Holy Spirit will be always working to convict and to convert any who are chosen to be saved.
As for those who are martyred (the “great multitude”), we may place the blame squarely on the Antichrist and the harlot (the false religious system). And this follows squarely, after so many years, to what happened in ancient Rome with the Roman emperors, and then with what happened with the evil Roman Catholic church. So, it will be repeated in the revived Roman empire. But I think it will be reversed: the Great Harlot will be the first to martyr the Christians, as we see in Revelation 17:6; then, when the Harlot is destroyed and the Antichrist is given full power (at the midpoint of the Tribulation), he will be the one to martyr the Christians. As we see in Daniel 7:21, and also in Revelation 13:7, “It will be given to him to make war with the saints and to overcome them.”
Please note what it says in Revelation 13:7; “It was given to him to make war with the saints…” (bold italic for emphasis). This indicates clearly that God will allow the Antichrist to martyr the saints, and thus to be in control.
Another interesting couple of verses that shows us this control is in Revelation 6:10-11. Here the martyred and resurrected saints were asking God, “How long, O Lord, holy and true, wilt Thou refrain from judging and avenging our blood on those who dwell on the earth?” And He told them that the time of killing (by the Antichrist) would go on “until the number of their fellow servants… should be completed.” In other words, God is in control. He will not allow anyone to be martyred beyond what he has predestined.
Please note also (from Revelation 7:10-14, 14:13-14, and 15:2-4), that God in the end will greatly bless the resurrected martyrs and give them victory and a song of joy and worship to God forever.