First of all I said “true and faithful” not just those who call themselves Christian. Second, let’s look at what was said to those churches.
Church 1.
Revelation 2:5 Remember therefore from whence thou art fallen, and repent, and do the first works; or else I will come unto thee quickly, and will remove thy candlestick out of his place, except thou repent. Repent or they will be removed.
Church 2.
Revelation 2:10 Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life. [warning them to be faithful.]
Church 3.
Revelation 2:16 Repent; or else I will come unto thee quickly, and will fight against them with the sword of my mouth. [Again told to repent or else.]
Church 4.
Revelation 2:22 Behold, I will cast her into a bed, and them that commit adultery with her into great tribulation, except they repent of their deeds. [Telling them that there will even be people within churches that will be cast out with some in the church who remain “faithful” will be ok.]
Get the drift of what’s happening here? There will be some whole churches who will be cast out and there will be some within churches who will be and the last church, the church in Philadelphia, is the example of the true, faithful church and where, once again, we have the promise of not having to go through the tribulation.
Revelation 3:10 Because thou hast kept the word of my patience, I also will keep thee from the hour of temptation, which shall come upon all the world, to try them that dwell upon the earth.