Which is the real god? is there any harm in praying to any god irrespective of various religion?

Which is the real God? How do we realise his presence, some people talk about rebirth and gods presence? what is the way to realise his presence?Is all lthe god One?is there any harm in worshipping different God?If God really existed why do we have calamaties and Crimes? Which religion speaks the truth andwhich is true?