There's a subtly in this verse of great significance.
God has introduced Himself with a new name, Yahweh. So He clarifies to Moses that the Patriarchs, Abraham, Isaac and Jacob didn't know Him by this name. They knew Him as God Almighty... or El-Shaddai, the All-Suffient God.
Now that may not seem significant. It may sound like a splitting of hairs, but understand that for the Patriarchs, God was all about the covenant, or the promise. And these three men, as the verse reminds us, all died not seeing the promise fulfilled. They lived in Canaan as foreigners.
God told Abraham long ago that he and his descendants would inherit the land. If that were me, I'd kind of think sometime during my life, I'd see the promise come to pass. But three generations came and went... then 430 years of slavery to the Egyptians... then 40 years wandering in the wilderness... before the Israelites took possession of the promise land under the leadership of Joshua.
So what IS significant about this name change, is that Yahweh is the God performing what He promised. While Abraham, Isaac and Jacob were noted for their faith in the covenant, they didn't see any earth shaking miracles. But Moses is about to see some nature changing stuff in order for God to show His power and free His people. THAT is the difference between El-Shaddai (God Almighty) and Yahweh... not so subtle any more.
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