They are not "in Christ" for they believe in some other Christ, not the Christ we believe in who is our Lord and God.
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Have you read the Gospel, ad-o?
I'll let you look for yourself without quoting chapter and verse.
However, here are a few pointers to get you started.
1. The bit where Jesus tells His followers that there are 'other flocks' - in other words, not part of the Galilee set.
2. The bit where Jesus went out of His way to work with those the religious teachers thought were 'heretic'.
3. When the disciples complained to Jesus that others were doing stuff in His name, what did He say?
Let me envisage a scene.
The Pearly Gates.
A chap comes along and he's let in.
Can you imagine St Wogburger the flatulent grabbing Christ by the collar and saying -
"Hang on; that guy hasn't went through the ritual, had insence stuck up his nose, been half drowned in oil an even worse, he can't be bothered with liturgy stuff! Why should You let him in here?"
"Well", I can hear Christ saying, "I love Him - and Martin Luther King's a good guy who loved Me."