How close is the day when the trumpet call takes believers out of this existence and changes them?

The day is getting closer and closer. The trumpet call will take believers out of this existence, and they will suddenly be changed, as Paul says. No exact date is given; the context emphasizes increasing nearness amid intensifying persecution and warfare.

Les explains that just as a Roman trumpet call signaled victory and going home, the believer's trumpet call will signal the battle is over and we're going home. He says we are in a fierce battle, worldwide persecution against Christians is rampant and will get worse, but we're getting closer and closer to the day when that trumpet call takes us out of this existence and we are suddenly changed, as Paul says. He then directs us to the corresponding portion of Scripture for the full picture.

1 Corinthians 15:51

Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,

What happens to believers when the trumpet sounds?

When the trumpet sounds, believers receive the call that the battle is over and they are going home. They will be taken out of this present existence, and, as Paul says, they will suddenly be changed. This call signals victory and the end of the warfare.