One reason so many people doubt Dumbo, a/k/a Dim Won, b/k/a Barack Obama is a Christian is that he does not act like a Christian, From Glenn Thrush, Politico: 
[NBC News Brian] Williams, sitting under a tent in a rain-soaked New Orleans, where the First Family commemorated the fifth anniversary of Hurricane Katrina, asked Obama why so many people were uncertain about something so fundamental as his faith.
“I can’t spend all of my time with my birth certificate plastered on my forehead,” quipped Obama, who took a deep breath to gather his thoughts when asked if the poll reflected his inability to communicate with voters
When questioned about faith, what would Jesus do? Christian convince skeptics by displaying Christian virtues. When questioned about his faith, Dumbo acts like well an infidel. As I understand Christian faith, if Dumbo accepts Jesus Christ as his savior, he needs no others blessing.