There are a variety of posts like the one in the OP that claim the song is intended as a Christian anthem. For example here, here, and (indirectly) here. However, Everything I've found directly attributable to Groban (that is, with a verifiable citation) gives no indication of what he has in mind when singing the song. He has acknowledged that he was raised Episcopalian and that his father's side of the family is/was Jewish -- this is documented in a variety of places, like Wikipedia -- but beyond that, he seems not to be publicly religious.
My own conclusion is that Groban, by intention, makes no claim about how the song should be interpreted, leaving that up to each individual.