Fervour is the noun: the strong feeling itself, not the person who has it. Religious fervour, patriotic fervour, the fervour of a crowd singing together. It is heat of belief or emotion, not a mild liking. This spelling with -our is the British one; American English writes fervor. Both feel a little formal. The word does not tell you whether the cause is good. You can admire someone’s fervour, or you can worry about blind fervour — feeling so hot that thinking goes quiet. In older writing you also meet it for actual heat, but in modern English it is almost always about feeling.