Graham, who is the son of Billy Graham and the CEO of the Billy Graham Evangelistic Association, said on MSNBC's "Morning Joe" that Obama "has said he's a Christian, so I just have to assume that he is."
"All I know is I'm a sinner, and God has forgiven me of my sins... you have to ask every person," he said about whether he could say for sure that Obama is indeed of the Christian faith.
However, when asked about GOP presidential candidate Rick Santorum's religion, Graham gave a much more concrete answer.
"I think so," Graham said when asked if he believes Santorum is a Christian. "His values are so clear on moral issues. No question about it... I think he's a man of faith."
MSNBC's panelists questioned the reverend's double standard, but Graham continued to draw distinctions between the candidates on the issue of faith. On Mitt Romney, Graham was again evasive, stating that "most Christians would not recognize Mormonism as part of the Christian faith."
But Graham was more willing to label Newt Gingrich's faith. "Newt's been married several times... but he could make a good candidate," Graham said. "I think Newt is a Christian. At least he told me he is."
Later in the segment, Graham also said he could not be sure that Obama was not a Muslim.
"All I know is under Obama, President Obama, the Muslims of the world, he seems to be more concerned about them than the Christians that are being murdered in the Muslim countries," he said.
He continued, "Islam sees him as a son of Islam... I can't say categorically that [Obama is not Muslim] because Islam has gotten a free pass under Obama."
Graham drew the criticism of the White House last spring when he suggested in an interview with ABC that Obama had not been born in the United States.
During that same interview, Graham also questioned whether Obama's actions and values matched up with his identification as a Christian.
"Now he has told me that he is a Christian. But the debate comes, what is a Christian?" Graham said of Obama. "For him, going to church means he's a Christian. For me, the definition of a Christian is whether we have given our life to Christ and are following him in faith and we have trusted him as our lord and savior."
Watch Graham's full interview on MSNBC: