I'm thinking a religious UA, we can't give them a free belief, but how about giving them the power to gain a second religion at the cost of having two simple religions, each with the same pantheon belief, one founding belief, one follower belief, and no ability to be enhanced in any way? Their first Great Prophet founds the first religion as usual, their second Great Prophet, instead of enhancing the first religion, can found a second religion, and from their the rest of the Great Prophets act as normal, with no ability to enhance either religion.
Would this be considered overpowering?
It may not be overpowered, but I think it would be difficult to manage, requiring some changes as to how religions work.
I think a good UA could be
Temple of David: Each religious building provides 1 faith, 1 culture and 1 gold to the capital
Or maybe
Ark of the Covenant: Great Prophets provide a 15% combat bonus to nearby land units, and damage adjacent enemy units by 10hp per turn {or perhaps evoke terror in the enemy}
or in the spirit of Dido and her elephants;
Split the Sea: Units can enter and exit the ocean without movement penalty and attack without penalty from the sea against land units while there is a great prophet on the map
or
Exodius - If you are defeated in the ancient or classical era, you will return in the modern era
or more seriously,
Divine Inspiration - All units gain a 20% combat bonus while there is a great prophet on the map. That one might make a good belief actually... Maybe make an extra 20% combat bonus while fighting near your holy city or in the vicinity of a holy place (settled prophets).
As you can see, I think that any ancient Israeli civ should use prophets in a unique way