I've been presented with questions like this before, as have many of my colleagues. As a physics grad student I work in a pretty diverse environment, we have lots of Hindus, Muslims, Buddhists, Christians, Jews, and atheists, it's nowhere near as uniform as some might think. The answer I normally give, as well as my colleagues, is that "Let there be light" doesn't really say very much. As scientists we ask, "How much light? At what temperatures, what energy densities? What happens as that light spreads out and cools? How long did it take? How long ago? How fast are the galaxies flying apart as a result?"
The Big Bang (which itself is represented by multitudes of different models) is an extremely detailed mathematical account of the conditions under which the universe as we know it began. Not only do these models spin experimental measurements into a very long, very precise story about the early universe, but having such a story gives you the ability to make predictions about properties the universe ought to possess today.
Cosmic microwave background radiation is probably the most famous triumph of the Big Bang theory, as Big Bang models were used to predict both the existence of this radiation as well as its temperature, well before it was ever first detected or measured. "Let there be light" doesn't give you any of that predictive power, it can mean any number of things.
There's also the issue of dates and ages and things. If you have a very loose interpretation of Biblical details, it doesn't need to contradict any facts known to scientists. On the other hand, if you want to be strict about details and ages and things, going with notions such as a 5000-10000 year-old Earth, this is where religion and science can start to conflict. Besides, the Big Bang theory isn't set in stone- we can say with high certainty what the universe looked like 13 billion years ago (i.e. extremely hot, extremely dense) but we can't say for sure whether time truly began at this moment, or whether there's more to the story.
