If you fervently believe or feel something, then you believe or feel it strongly — with a lot of passion.
People feel fervently about a lot of things. A sports fan may be fervently supportive of a football or basketball team and go to every game. Many religious people fervently believe in a god. If you're extremely self-confident, you might fervently believe in yourself. This word means a lot of passion is present. If you feel blah about something, or you just like it a little, you don't fervently believe in it.
