Possible solutions;
1. It is not Hezekiah's seal, but his father's seal and archeologists are not reading what it is saying correctly.
Ahaz, Hezekiah's father was a very idolatrous king.
2. It is Hezekiah's seal, a gift from his father or maybe someone in his court (the seals of some of Hezekiah's advisors was also found with it), but he threw it in the garbage.
3. The most unlikely option considering what the Bible say about Hezekiah, is that he had the seal made, and the seal ended up in the garbage by accident, or was thrown there when
Manasseh took over as king. Manasseh was more like his grandfather than his father, and it seems to me that he would have no issue with Egyptian gods.
The seal was found among garbage. Someone threw it there. A king's seal? Why?
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