but whether it is really important if I know
from the OP. I relate to that -- it was one of the conclusions I came to, too, on my journey toward rationality. I realized one day it doesn't matter whether I believe or not, belief doesn't change reality. I realized it didn't matter whether or not I could know for certain about god. I decided to just get on with life. Eventually I came to another realization -- that those things that ARE proveable and knowable, stuff scientists in all fields have learned and shared, make it extremely unlikely that god exists. All the different gods that people believe in are just reflections of themselves.
As for challenging the beliefs of others, I mostly don't unless they insist on preaching. If you bring up a subject, I have the right to ask questions. I'm not personally attacking anyone, just asking questions about the beliefs they have preached to me. If you don't want an atheist to challenge what you believe, quit preaching about it to them. Or ignore them.