Q. During the Great Tribulation, if a believer and/or his loved ones are tormented (in whatever broadest sense) to the point he denies Christ, will he lose his Salvation?
A. The way I understand it, Tribulation believers will be expected to die rather than officially deny the faith. The people tormenting believers will be the forces of the anti-Christ in support of the one world religion. According to Rev. 13:16 they will require everyone to take the mark of the beast and according to Rev. 20:4 refusal to do so could result in their execution. Rev. 14:9-11 explains that taking the mark will make salvation impossible. This is because taking the mark means swearing loyalty to the anti-Christ
All that said, I think there will be many who will find a way to evade the officials and avoid taking the mark. They’ll have a difficult time and will truly have to live by faith to do so, but I’m convinced many will find a way. Otherwise there wouldn’t be anyone left to re-populate the Earth in the Millennium.