I have always understood that a generation was 70 years as shown in Psalms 90:10 and Isaiah 23:15. But during my reading of the bible I came across a couple of verses in Genesis 15:13-16. It tells in verse 13 of Abram’s descendants being enslaved for four hundred (400) years and in verse 16 it refers to Abram’s descendants will come back in the fourth (4) generation. I take this to mean that a generation is 100 years. Is my thinking correct?
First you must understand that Psalm 90:10 and Isaiah 23:15 don’t describe the length of a generation, which is the span of time from the birth of a man to the birth of his first child, nominally 40 years. Instead they establish the length of an average man’s life.
Second, Genesis 15:13 says the Israelites would be enslaved in Egypt for four hundred years. Then Genesis 15:16 says in the fourth generation they would come back to the place where Abraham was standing, which was in the Promised Land. The two references can’t be related because men did not wait to reach 100 years of age before starting a family.
Here’s what happened. The Israelites did spend 400 years in Egypt. Near the end of that time the Lord sent Moses to be their deliverer. His birth marked the beginning of the first generation of the deliverance. But when he was 40 years old he killed an Egyptian soldier who was mistreating an Israelite slave (Acts 7:23). As this happened the second generation was being born. He fled into the desert of Midian and hid for 40 years (Acts 7:30) until everyone who had known about the murder was dead (Exodus 4:19). When the Lord sent him back to Egypt he was 80 years old (Exodus 7:6) and the third generation of people was being born.
Moses led the Israelites toward the Promised land. But they rebelled against God and were required to spend 40 years in the wilderness, a year for each day the spies had spent in the Promised Land (Numbers 14:33-34). God said everyone over 20 years of age would die there (Numbers 14:29). During their time in the wilderness the fourth generation was born. And just as the Lord had said, in the fourth generation the Israelites entered the promised land under the command of Joshua, who had succeeded Moses as their leader.