Kengor concludes that Obama, raised by apostates and atheists, probably has little faith. On the basis of very modest evidence, I am inclined to agree.
I imagine people suspect Obama is a Muslim because he is more accepting of Muslim people and Muslim culture than are most Americans, having lived with them as a youngster named Barry Soetoro.
What of his attendance at the church of the Rev. Jeremiah Wright as an adult? Raised in Indonesia and Hawaii, by whites and Asians, I suspect Obama attended Rev. Wright's church mostly to learn how to successfully imitate an African-American. Of this I have no proof, but it seems logical.