Translation:If you don't believe, the spell will not work.
כישוף would be better translated as witchcraft rather than spell. I think. Not sure.
You may be right. I'd say spell is לחש. But I'm not an expert on witchcraft and wizardry. :-)
I'm not either, but I looked up "spells and wizardry" in Morfix so as to get the right words first. For spells, the words were: כִּשּׁוּף, לַחַשׁ, קֶסֶם