Was Israel the only nation that knew the one true God?
Yes. Israel was the only nation that knew the one true God. Every other people group in the then-populated world was steeped in paganism and polytheism, worshiping many gods. Israel alone, as God's separated covenant people, had knowledge of the one true God.
Les explains that ever since the Tower of Babel, the whole human race until Abraham was saturated in paganism and polytheism—the worship of many gods. When Israel came on the scene as God's separated, different covenant people, they were the only people on earth with a knowledge of the one true God. The populated world at that time, along the Mediterranean area and into the Middle East and maybe out to China, was entirely pagan worshipers of many gods, except those coming from Abraham, Isaac, and Jacob. Les also notes that every plague in Egypt was directed against one of Egypt's gods, proving their pagan worship had nothing to do with Him and He could destroy them at will.