When you sacrifice the present for a possible better future, you have faith in that process.

For example, if you don't eat a pizza today, you hope that you will be thinner in the future. You don't know 100% that it will happen, many things could happen in your body, but you hope for that to happen.

That's faith.

That's all.

It doesn't require the existence of a God.

The whole faith thing is simply that, if you behave nicely in the present, you will have a great future (heaven). And if you behave badly in the present, you will have a bad life ahead (hell).

As I see it, heaven and hell are not places after you die, but simply your future, in this life. So, if you want to get to heaven (have a good life in the future), you have to make sacrifices in your present. If, on the other hand, you only do vices and are a bad person, you will end up in hell (you won't have a great life in the future).