I also believe that “Christians” will experience some of the tribulation.
There are many self-proclaimed “Christians” in the world that honestly believe - either by deception or ignorance - that they are saved. Scripture that you have recently quoted (Matthew 7:21-23) confirms this. Given this truth, I believe that the Lord will use tribulation, and “the mark" to allow all self-proclaimed “Christians” the opportunity to positively know whether they are saved or not.
And speaking of “the mark” - I don’t believe that we are too far away from it. The only thing standing it the way is cash.