In Southern Baptist churches you cannot “join” unless you are professing faith in Christ as Lord. I don’t know about other denominations. Perhaps they do things differently. I think the Presbyterian reformed way is to assume you are until you prove you’re not, or something like that. Not trying to misrepresent anyone’s denomination here, so anyone reading this don’t jump on me. Just guessing, but I do know that one cannot join an SBC — i.e. become a member — unless one professes at the same time — faith in Christ as Lord — or by letter from another SBC church (which would be the same thing since one could not have joined THAT church unless following the same process).
Most churches I know have the same policy, that you have to profess to be a believer in Christ.
Still, there will be those who aren’t who get in either through deception or being raised in the church they know the right lingo and think they’re saved and they’re not.