What is the significance about the Danes praying to the old stone gods in "Beowulf"?
One of the many themes in "Beowulf" is that of religion. In this and many other Anglo-Saxon works, there will be evidence of both pagan and Christian elements since Christianity was spreading rapidly, but it makes sense that people didn't give up their habits of pagan worship (multiple gods and idol worship) overnight. This is one element that is absolutely characteristic of Anglo-Saxon literature, which is why it is significant.
In Beowulf, there are many instances of both--some examples of the pagan beliefs include worship to stone gods, references to Fate controlling Grendel's actions and putting Beowulf in certain positions, the dependence on "lucky" items such as Hrunting or the sword in Grendel's mother's lair which is used to kill her and behead Grendel. Christian references included, but are not limited to, the throne of Hrothgar which is protected by God, references to Heaven, the image of Beowulf outstretched in the form of a cross in the lair of Grendel's mother, the way the lair and the waters light up the minute Grendel's mother is killed to symbolize the goodness of God on Beowulf's side as opposed to the dark, evil lair it was before she is conquered.