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for that. I was told, apparently mistakenly, that it was a terrible thing to call non-Muslims kaffir, in case they later reverted to Islam, and then the word I used for them would "bounce back" on me and I would be damned as a kaffir.

Assalam o alaykum,

This so called argument is faulty. Then one shouldn't refer to a Muslim as a Muslim because he might leave Islam in the future.

How about "liberal" Muslims who don't deny Allah but who don't follow the teachings of the Quran, the fardh like salah, and who openly commit sins like drinking alcohol, what are they called?

As long as they don't deny any of the fundamentals of Islam, they will be considered Muslim regardless of how many sins they have committed. Committing a major sin doesn't take one outside the fold of Islam unless it is considered to be permissible (halal).