Nephi in Chapter 14 vs 10 quotes an angel as saying "Behold there are save two churches only; the one is the church of the Lamb of God, and the other is the church of the devil; wherefore, whoso belongeth not to the church of the Lamb of God belongeth to that great church, which is the mother of abominations; and she is the whore of all the earth."
If we view Nephi's use of the word "church" in terms of: Catholic, Protestant, Presbyterian, Lutheran, Latter Day Saint or any "other" religion, or denomination, we may miss the extent of the teaching. We would be speaking in terms of earthly legal institutions, instead of the qualities and classifications that make something of the Devil vs of the Lord. Anyone belonging to any earthly church could subtly and carefully be a part of "the church of the devil". He's a master of deception after all. So when we say the great and abominable church is "them" (and therefore not us) that almost blinds us from the benefit the verse can offer us.
Jacob Nephi's brother says this : 2 Nephi 10:16 "Wherefore, he that fighteth against Zion, both Jew and Gentile, both bond and free, both male and female, shall perish; for they are they who are the whore of all the earth; for they who are not for me are against me, saith our God."
That abominable, and all-inclusive great church could be comprised of all philosophies, belief systems, unbelief systems, rationalizations, theories and vanities that distract people from repenting and following Christ. This all-inclusive church is a "whore" because she is open to all. It's welcoming. Your denomination doesn't matter. The only requirement being that you have false beliefs. The words are pretty brutal, if your not part of the Church of the Lamb, your a part of that other church This is big stuff that makes me look very closely at myself. Again if we speak in terms of earthly classified denominations we may not be using the same criteria as the scriptures to identify a "Church". But if we classify "church" more broadly, we may see that we too have some repenting to do regardless of where or in who's database we have membership records.
Having your name on membership records of an earthly institution we call a church may not mean much if internally we have adopted a false belief system, or follow false teachings and thus become part of a false church. You could be a member of any church, but not follow the Son of God and therefore in reality believe things that go against God. I think this is why the scriptures warn everyone to repent. The safety is in coming to Christ, and as Lectures on Faith states: "having a knowledge that your life path is acceptable to God". Not just thinking you are part of some organization which is "safe".