The Mekhilta deepens the significance of Yithro's confession by pointing out that he was uniquely qualified to make it. "There was no idolatry in the world that Yithro did not come across and did not worship." He had tried every religion, served every god, performed every ritual. His declaration that the Lord was greater "than all the gods" carried weight precisely because he had personally tested all the alternatives.
But the rabbis placed Yithro in a larger pattern of outsiders who recognized God's supremacy. Naaman the Syrian general was even more conceding than Yithro. After being healed of his leprosy by the prophet Elisha, Naaman declared (II Kings 5:15): "Now I know that there is no God in all the land, but only in Israel." Where Yithro said God was greater than other gods, Naaman went further — there is no God at all except in Israel.
And Rachav the harlot of Jericho surpassed even Naaman. When she hid the Israelite spies, she told them (Joshua 2:11): "For the Lord your God is the God in the heavens above and on the earth below." Rachav's confession extended God's sovereignty beyond the land of Israel to encompass all of creation — heaven and earth alike.
Three outsiders, three declarations, each more expansive than the last. The people who had the least reason to believe became the ones who articulated belief most powerfully.