"Я християнин, а мій друг - мусульманин."
Translation:I am a Christian, and my friend is a Muslim.
They're just different parts of speech. Християнин is a noun referring to a Christian person (specifically a male, feminine is християнка, plural is християни).
Християнський is an adjective, so you'd use it when saying something like християнська віра (the Christian faith), християнський рок (Christian rock), etc.