God was promising Abraham the world, or at least a large chunk of the known world currently occupied by the Amorites. But he wouldn’t get it just yet:
Genesis 15:13–16You can be sure that your descendants will be strangers in a foreign land, where they will be oppressed as slaves for 400 years. … After four generations your descendants will return here to this land, for the sins of the Amorites do not yet warrant their destruction.
He would have to wait some 400 years.
(The word ‘generation’ could just as easily have been transcribed as ‘age’, ‘era’, or ‘time’; so there’s no point in arguing over who was where and when.)
There’s some discussion about when the 400 years started: was it with the birth of Isaac, or the promise of the birth of Isaac, or while Abraham himself was in Egypt, or when Ishmael started bullying the five-year old Isaac, or of when Joseph brought his family across to Egypt?
Paul (Galatians 3:17) and Moses (Exodus 12:40) both report a sojourning time of 430 years for the sons or children of Israel. Sojourning could mean anything, and there is an argument that Israel as a nation started with Abraham, when God promised him the earth, or at least part of it, and that he would be the father of the nation to eventually populate it. That promise is a neat 430 years before the Exodus.
You can also see a neat four generations of the main players. The 64-year gap is covered, briefly, in just a few verses in Exodus 6.
|Generation||Elapsed time||What happened|
|Abraham||0||God makes his first promise to Abraham (Gen 12:1-4)|
|25||Isaac is born (Gen 21:5)|
|30||Ishmael starts bullying Isaac (Gen 21:8-9)|
|85||Esau and Jacob are born (Gen 25:7)|
|100||Abraham dies (Gen 25:7)|
|176||Joseph is born|
|193||Joseph is sold into slavery in Egypt (Gen 37)|
|205||Isaac dies (Gen 35: 28–29)|
|215||Joseph brings his family, including his father Israel, to Egypt (Gen 45:4–6, Gen 49)|
|232||Jacob dies (Gen 47–49)|
|Joseph||286||Joseph dies (Gen 50:26)|
|A gap||This 64-year gap could well have been the time of severe oppression in Egypt.
|Moses||350||Moses is born (Ex 6: 16–20)|
|430||The 80-year old Moses and his brother Aaron ask Pharoah to let God’s people go. They leave shortly afterwards. (Ex 7:7, 12 and Gal 3:16-17)|