Someone can be a "Christian", a believer in Christ, and not necessarily believe that they know they're right about it, nor believe it is the only possible correct religion, or necessarily believe every word of the Bible is true.
Someone else can be a Christian and be a Catholic and have very strict adherence to a very strict doctrine.
Someone else can identify as Christian on a census form and be fairly apathetic about the whole thing.
Yet all three of these people are counted as Christians.
I consider folks who were never taught a religion and don't believe in the supernatural to be atheists by default. I consider folks who aren't aware of the supernatural to be atheists by default. Their views may not mesh with strong atheism, but they CERTAINLY do not mesh with being a theist, and most closely resemble atheists in practice, if not in 'spirit', pardon the pun.
I don't think the point you're arguing is worth fighting for, frankly, because the implications of such would probably mean I can say half of all Christians aren't really christian.