I think this is addressed early in the movie, when two Army officers are telling Indy and Brody about the Nazi communiqué they've intercepted. They ask Indy what the Ark looks like, and in response (according to the script):
Indy pulls a big format book from the stack on his lectern and flips through the pages until he finds a large color print. […] two men carry the Ark of the Covenant, a beautiful gold chest, crowned by two sculptured gold angels. […] the most astounding thing in the picture is the brilliant jet of white light and flame issuing from the wings of the angels. It pierces deep into the ranks of the retreating enemy, wreaking devastation and terror.
One of the officers asks:
MUSGROVE: What's that supposed to be coming out of there?
INDY: Who knows... lightning... fire... the power of God.
That seems a fair description of what we see when they open the Ark at the end of the movie.