The word “Christian” is being used one way by Catholic posters, & another by non-Catholic posters…
To many Protestants,( not just OSAS people, either), a Christian is someone who is what most Catholics refer to as “a Christian in the state of grace”.
To those same non-Catholics, saying someone who was baptized as a child, but who may now profess no faith in anything, is a complete confusion of terms…An evangelical Christian means by the word Christian, specifically someone who has a deep commitment to Christ.
Yes, you are quite correct that people apply the name Christian in very different ways. I believe that C.S. Lewis actually wrote about the use of the word Christian in one of his books, but I don’t recall exactly where at the moment. Probably “Mere Christianity”.
Anyway, my practical definition, is that I generally call anyone a Christian if they claim to be one, that is they claim to be a follower of Jesus and to live according to the faith he taught, or if they have been baptised and have not explicitly rejected the faith. This means that there are many nominal Christians (by this definition) who might seldom attend religious services, many who’s theology could be described as heretical, etc.
If one wants a more narrow definition, you could extend this to specify that a Christian is someone who agrees with any interpretation of the apostle’s creed, just to eliminate oddities like someone who imagines that Jesus was a Martian, reincarnation of Buddha, or whatever.
If there is a need to be more specific, there are plenty of adjectives to make more clear what kind of Christian I am referring to, e.g. orthodox (one with theology that is sound & traditional), devout (one who regularly participates in religious activities such as going to church, prayer, scripture reading, etc.), Catholic (in communion with the See of Rome), consecrated, “in a state of grace”, etc.
Using this definition, one might be a Christian at one time and no longer, but I would not say someone is a non-Christian because they are sinning, or because they have not been to a Church in a long time. Rather, I would say they are no longer Christian when they indicate clearly by words or obvious actions (such as publicly adopting a different faith) that they have rejected Christianity.
Using the above definition, I think it is clear that not all such “Christians” will be saved.