You are reading the sentence incorrectly. He is not saying the president is a Christian. Read it again and look at the sentence clauses and structure.
If the comma was absent the implication would be one of Obama being Christian. With a comma present, "many Christians" and "the president" are separate entities.
Reread and looked at the structure.
I am now unsure exactly what the writer had in mind. As most of the MSM would have, in their typically bad usage, meant any number of things. As this is National Review, I usually give them the benefit of the doubt. I do tend to think that the participle dangles a bit.