Being a Christian has nothing to do with religious denomination. One can be of no denomintation. A Christian may live in a country where there are no Christian denominations of any kind. Denomination is irrelevant.
As far as I can see, denomination is relivent since there is no single enitity that can claim being the one true way, I'm sorry but I just don't see how denomination doesn't matter since a lot of them conflict on basic rules and on parts that are outside the NT
Christianity is a citizenship thing. Someone may be an infant Christianity-wise and hold on to all kinds of non-biblical view (or rather incorrect interpretations of what the bible says) but if they have been made a citizen it matters not. Citizenship is what counts. That act on Gods part which makes a person a citizen.
What defines "citizenship"?, this just seems arbitrary and less than a real answer
Take the old testament believers. They had no gospel doctrine to either agree or disagree with, Christ had not yet come. It doesn't matter. They are saved "in Christ" as much as the person today can be saved in Christ even though he lived 2000 years ago.
How would they be "in Christ" since they didn't believe he was anything but a man? how can you be in anything if you don't believe it? does belief suddenly not matter? I'm asking this because I know that like anyone, no one likes to have thier own beliefs dictated to them