A leaked email shows a staff member from the Democratic National Committee (DNC) wanted to question the candidate's faith because making out he was an atheist "could make several points difference".
Hillary Clinton will be named the Democratic nominee for US President at the party's conference this week after she saw off a challenge from Sanders.
At one point in the race between the two there was a fear Sanders could overtake Clinton leading to an email from DNC chief financial officer Brad Marshall.
He suggested that Sanders had been overplaying his Jewish roots and was in fact an atheist.
Comparing this to Clinton's committed Christian faith would gain her votes in key states, he said.
"It might may (sic) 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."
He has since apologised for sending the emails saying they were "insensitive".
"The comments expressed do not reflect my beliefs nor do they reflect the beliefs of the DNC and its employees. I apologise to those I offended," he said.