Well, he was a person who used violent means to achieve a political end. This drive was partly because of his faith/his interpretation of his faith.

If 'political designs sought for with violence' is the definition of terrorism, than John Brown was indeed a terrorist.

This sort of falls under the trope of "they're freedom fighters if we like them and terrorists if we don't" but...

Brown committed clandestine, violent acts in order to achieve an ideological goal. Even if you agree with his goal, if we are using the dictionary definition of a terrorist he fits the bill.