>"The repetition of behavior, not the sincerity of belief, is what shapes character"

To perform behavior X repeatedly and consciously for a long time, you have to have a belief (whether it is good or bad). Hence it is the sincerity of belief which shapes character.

Like when you wash yourself every now and then: you repeat that because you have a belief that keeping yourself clean is useful. Without that belief, you won't waste your time on that. Behavior is just an expression of a belief.