Miracles decks have been super strong in Modern, and the new Kefnet god practically turns all your spells into miracles. That's a huge power swing!
Besides that it can be used in Taking Turns decks and it has a very solid body, which is very hard to remove permanently.
The best part about Kefnet is that his ability doesn't use your spells, but casts copies of spells instead, which practically doubles the number of all your spells.
This card could definitely be broken in the right type of deck in Modern.