Personally there have been many times were I've achieved serenity after deep reflection and to ME there is only one answer as to why. Again, for ME it's about what I feel after doing certain things where I get that feeling no where else.

I'll give you an example:

You are a ghost hunter. You BELIEVE they exist whole heartedly. Some people don't agree with you for a bunch of different reasons. The fact that THEY don't believe doesn't change the fact that YOU believe. Why? Because you are satisfied that they do exist. This is the same way that some of us believe in God the way we do, and some people don't.

Some of the same reasons you believe in ghosts are similar to why some of us believe in God. Most of those reasons are really not 100% tangible, but they are still valid to you, right? The same with us.