I think that you need to repeat what you think the difference is.
1. something believed; an opinion or conviction: a belief that the earth is flat.
2. confidence in the truth or existence of something not immediately susceptible to rigorous proof: a statement unworthy of belief.
3. confidence; faith; trust: a child's belief in his parents.
4. a religious tenet or tenets; religious creed or faith: the Christian belief.
Belief
that implies definition 1, belief
in implies definition 3. I believe in my partner, for example. I can say that to him and it makes perfect sense. But if I say I believe
that my partner, it's gibberish. Something else is needed - I believe that my partner
will succeed, for example. To say one "believes
in God" is not necessarily to say that one believes
that God literally exists beyond a shadow of a doubt. Likewise, to not believe
in God is not to believe
that God literally does not exist beyond a shadow of a doubt. While one might sometimes not believe
in any gods and also believe
that no gods exist, it is only a logical overlap. It is not complete, seamless redundancy.