I think I know what Sagan meant, and perhaps "religion of science" was just a poor choice of words on his part. Where Sagan and others have excelled is to explain science in a manner understandable to most laymen, and at the same time project the awesome wonder and complexity of the universe as it is. I can't think of another, more appropriate phrase for this, but I'm sure someone has.

The sense of awe, what C.S. Lewis called "mythic joy," can be invoked by contemplation of the real universe as easily as by religion. If you can sit through "Cosmos" without shivers going down your spine, you're sleeping through the show.

Humans are built to reason, and to create new conclusions from data. The ecstatic feelings a scientist gets when some critical pieces suddenly fall into place, and the long-sought picture appears, the joy of bringing order to things, is part of the nature of being human.

"Epiphany" is not confined to religion.

My own suspicion is that religion is an outgrowth of this built-in desire, of early man's need to impose order on a seemingly unknowable complexity, more than religion is a source of it.