Yisro, Moshe’s father-in-law “heard of all that G-d had done for Moses, and for Israel his people” (Exodus 18:1, JPS). He journeyed from Midian to the encampment at Sinai, and brought with him Zipporah, the wife of Moses, and Gershon and Eliezer, the two sons of Moses. He proclaimed, “Now I know that the L-RD is greater than all gods” (Exodus 18:11, JPS). He implied, that in the same manner that the Egyptians conspired against the Children of Israel, so was Pharaoh and his army destroyed; i.e., measure for measure, by means of water.
Yisro, former minister to Midian, had worshipped many gods; and, according to Tanchuma, he had renounced idolatry many years ago. Yet, it was not until he heard of H’Shem’s plagues against Egypt – each one symbolizing H’Shem’s superiority over an Egyptian god – and the splitting of the Sea of Reeds, that he recognized H’ Shem as “greater than all gods.”
Up until then, his belief was predicated upon rational inquiry; he had his doubts about the efficacy of the many deities that he used to worship. Yet, when he heard of H’Shem’s greatness being demonstrated in a tangible way through the plagues, and the splitting the sea, his belief was upgraded to the level of knowledge, because of H’Shem’s miraculous intervention for the sake of Israel’s Redemption.
In other words, “seeing is believing;” although, it was enough for Yisro to “hear” “of all that G-d had done,” for his belief to become manifest. So strong was his belief in H’Shem, that he chose to align himself with truth. Only H’Shem is the One true G-d. All other so-called deities are no-things. Yisro’s fidelity shifted from potential to actual, when he travelled out to the encampment at Mount Sinai.
There, he presented offerings to H’Shem, and ate bread with Moses, Aaron, and the elders (Exodus 18:12). This ratified a covenant (agreement) between Yisro and G-d that effected a formal conversion, whereby he sought refuge under the wings of the G-d of Israel (see also Ruth 2:12).