I don't mean belief in a deity, but belief in yourself and your abilities.
As writers, we take a lot of hits. Harsh critiques from our peers, rejections from editors and agents, negative reviews, etc. And it's easy to get discouraged, to feel like you want to give up. But that's the worst thing you could do and that's where a strong belief comes into effect.
If you don't believe in yourself or your work, no one else will either.
It starts and ends with you!
So, you better believe!! Now please bear in mind that too much belief can lead to arrogance and illusions of grandeur.
Believe and be confident, but be reasonable, too.