Sanders is the Senator of Vermont, the nation's longest serving Independent Congressman, and the author of The Speech, about his 8.5 hour-long, near fillibuster on December 10, 2010 on the extension of the Bush-era tax rates.  Bernie was pro-civil rights in the 1960's, for gay marriage in the 1980's, and against the Iraq War from the beginning.
On the April 1, 2011 episode of 'Real Time with Bill Maher', the issue of pastor Terry Jones' mock trial and burning of the Qur'an which resulted in protests and numerous deaths in Afghanistan was brought up. Sanders says to Maher:
"Well, I think this calls for your second film on religion." [alluding to Religulous] 
Bernie is not actively involved in organized religion
“I am not actively involved with organized religion."
“I think everyone believes in God in their own ways. To me, it means that all of us are connected, all of life is connected, and that we are all tied together.”
Bernie gets some of his desire to help the less fortunate from some religious principles: “He often talks about ‘the ethical thing to do,’ ” said Gutman, “and his ethics are shaped by a concern for social justice and for other human beings that is part of a Judeo-Christian tradition.”
“I am motivated by a vision which exists in all of the great religions — in Christianity, in Judaism, in Islam, Buddhism and other religions — and which is so beautifully and clearly stated in Matthew 7:12,” Sanders said. “And it states: ‘So in everything, do to others what you would have them do to you, for this sums up the law and the prophets.’ ”
"Larry Sanders sums up his brother’s views this way: “He is quite substantially not religious.”"
The above quotes from The Washington Post.
Bernie indicates he doesn't believe in God
In October of 2015 Bernie Sanders was on Jimmy Kimmel live. Bernie didn't profess a theistic belief, indicated he didn't believe in god, and promoted the ideals humanism. He was referred to as a cultural jew which by many accounts is a non-believer who feels a kinship with the jewish people and religion whether that be by celebrating certain holidays or by acting what they perceive as "jewish." Celebatheists site owner and well known atheist Brian Sapient has referred to himself as culturally jewish, as well as American Atheists President, David Silverman.
Jimmy Kimmel says "A moment ago you said God forbid, you say you're culturally jewish, do you believe in god? Bernie instantly says, "No" while Jimmy is mid sentence
Jimmy goes on to ask: And do you think that's important to the people of the United States?
Bernie Sanders replies: Well I am what I am, and what I believe in, what my spirituality is about, is that we're all in this together. I think it's not a good thing to believe that as human beings we can turn our backs on the suffering of other people.
This is not Judaism, this is what Pope Francis is talking about, that we can not worship just billionaires and the making of more and more money. We are living in a nation today that has seen the proliferation of millionaires and billionaires, massive income inequality. And you know what? We have the highest rate of childhood poverty of any major country on Earth.
We are the only major country on Earth that doesn't guarantee healthcare to all people as a right. The only country that doesn't have paid family and medical leave. Many other countries are providing free tuition in their public colleges and Universities. So essentially what I think is, we do best as human beings, we fulfill our lives when we work together rather than say "hey I want it all, and I don't care about the hungry kid down the street." I don't think that's what America should be about.
Email from DNC CFO Brad Marshall
While Bernie Sanders ran against Hillary Clinton there were people within the Democratic National Committee working to ensure Hillary Clinton received the nomination instead of Bernie. In this email from Brad Marshall we see he believed that Bernie may be an atheist.
It might may no difference, but for KY and WVA can we get someone to ask his belief. Does he believe in a God. He had skated on saying he has a Jewish heritage. I think I read he is an atheist. This could make several points difference with my peeps. My Southern Baptist peeps would draw a big difference between a Jew and an atheist.