Why did God wait 400 years to give Abraham the promised land?

Abraham believed God as to his seed, and it was “counted unto him for righteousness” (Genesis 15:6), but when promised the land he asked for a sign. Would he not have had it at once had he believed, without having to wait four hundred years for his descendants to inherit it?

I do not think there was necessarily any unbelief in Abraham’s asking whereby he should know that he should inherit it. God often dealt thus with his servants of old—with Moses, Gideon, Hezekiah, etc., and granted them signs to strengthen their faith. As a matter of fact, Abraham did not ask for a sign. His question brought out the basis on which God was acting, namely, on the ground of covenant. The heifer, the she-goat, and the other sacrificial victims, all spoke of Christ the One true covenant Victim. Verse i8 makes it clear that the terms of this covenant assured the land, not personally to Abraham, but to his seed, and this was, we need not doubt, the original purpose of God. Stephen understood it so, for he emphasises the fact that “God gave Abraham none inheritance in the land, no, not so much as to set his foot on” (Acts 7:5), nor does He in any way attribute this to Abraham’s unbelief. It would clearly have been premature for a single family to inherit a vast territory, like that indicated in Genesis 15:18-21; nor had God’s time come to expel the owners of the soil, for “the iniquity of the Amorites was not yet full.”