Ksav Sofer explains that Yisro had originally been confident that the faith he arrived at independently, through critical inquiry, could never be swayed. Yet, after Yisro saw how Bnei Yisrael, who had witnessed first-hand the miraculous hashgacha pratis of Yam Suf splitting, in short time became rebellious and lax in their dedication to Torah to the point that Hashem allowed Amalek to attack, he was forced to change his mind. If those who saw miracles with their own eyes could fall so quickly, what chance did he have, living alone in Midyan surrounded by idolators?
I wanted to share with you a different answer I saw quoted in the name of R’ Elyashiv. When Moshe directed Yehoshua to select soldiers to fight Amalek, he used the expression, “Bechar lanu anashim.” Even though Moshe was far greater than Yehoshua, he spoke to Yehoshua like an equal and made it sound like a joint effort between them. Rashi quotes from Chazal that from here we learn that one’s student’s honor should be as dear as one’s own. Chazal also tell us regarding the splitting of Yam Suf that the vision seen by the lowest maidservant was greater than that seen by the navi Yechezkel. We learn from these episodes that the lowest person on the totem pole has the potential to be as great as the highest prophet; the student deserves as much honor and respect as his teacher. Yisro heard and saw that Judaism does not discriminate; being a ger or a servant or some other “nobody” doesn’t mean anything when it comes to avodas Hashem and the respect each individual deserves.