God does not condition His fulfilling this promise on any expected behavior on the part of Abraham. Its fulfillment is not dependent on Abraham's doing something in the future. This promise, unlike the promises in the later books of the Pentateuch, is an unconditional promise.
Consider, as a second example of an unconditional promise, Genesis 12:7: "Then the LORD appeared to Abram and said, 'To your descendants I will give this land.'" That is all there is to the promise. God attaches no ifs, ands, or buts to it at all. God simply says, in effect, "I will do it. Period."
The same could be said of any of the promises to the patriarchs. An analysis of Genesis 12:1-3, 7; 13:15-16; 15:18-21; 17:6-8; and 35:11-12 will yield this conclusion: In every single instance, the fulfillment of the promise does not depend on any future action or behavior God expected on the part of Abraham, Isaac, or Israel (Jacob). All of these scriptures record unconditional promises.
In making these unconditional promises, God revealed His purpose to the patriarchs, at least in outline. It is a purpose to which God is absolutely committed. He will not allow anything—or anyone—to stand in the way of His executing it. A good example of His resolute determination to carry out His purposes, no matter what individuals may do or think, is an incident which took place as God was about to lead the children of Israel into Canaan.
Fearful of the indigenous population, the children of Israel refused to enter the land—refused, in effect, to believe that God meant what He said when He promised Canaan to their ancestors, Abraham, Isaac, and Israel. In their rebellion, they even determined to "select a leader and return to Egypt" (Numbers 14:4). God's people, lacking faith, were actually trying to thwart His purposes. He was so angry with their lack of faith that He thought to "strike them with the pestilence and disinherit them, and . . . make [Moses] . . . a nation greater and mightier than they" (Numbers 14:12). To fulfill His unconditional promises to Abraham, God was willing to destroy an entire people and raise up another through Moses, through whom He could honor His promises to the patriarchs.
As Numbers 14:13-20 indicates, Moses dissuaded God from taking such drastic action. Nevertheless, the episode illustrates the zeal God displays in honoring His promises. He means business.