Belief in Neopaganism?

I can understand somewhat why Christians, Muslims and Jews can claim to be true since there history continues back so far. I was wondering how someone who identifies as neopagan finds there belief and the means to justify it? I wonder this seeing (at least in my limited understanding of neopaganism) it is based on the pantheons of pre-Christian religions like that of the Norse, Celts and Ancient Greeks. I have read extensively on these old religions and so much information has been lost through time. How do you know then that the way you worship is right? And why was it not until the 19th century that they started being worshiped again? I'm not trying to attack your ideas or anything like that, I am simply curious and would like to understand neopaganism better.