[Chai.]

(1) If you want valid proof of God's existence, well there is none. Believing in God depends entirely on faith. Some people are brought up to believe in God and they go on doing it all through their lives. Some start believing in God after incidences that happen to them or in their life.
Born in a traditional Hindu family and educated in a convent school, I always had this question about God's existence. But a few incidences (I would rather not share them here) made me start believing that God is there. I cannot give you a valid proof about God's existence and I cannot force you to believe in God. It all depends on you and your choices and faith. Yes, nobody has seen God, but yes, I feel that there is God and yes, I do believe in God.

(2) I am not a christian, like I mentioned above. But, I don't really think everyone who believes in God or follows a particular religion has negative feeling about people from other religion or people who chose not to follow any religion.
Yes, there are extremists everywhere,in every field. People with the attitude that only their beliefs are correct and the world is wrong. But, you cannot change their thinking. So, it is not all 'religious people' as you say.

I know my posts do not answer your question, but I don't think anyone can give you a valid proof/answer. One more thing I would like to add is, I always believed that God is one and people see him and call him with a different name.
So well, yeah, that is it.