Genesis 22.17-18:
I will surely bless you, and I will multiply your descendants like the stars in the sky and the sand on the seashore. Your descendants will possess the gates of their enemies. And through your seed all nations of the earth will be blessed, because you have obeyed my voice.
Context: Many Christians understand the promises that God makes to Abraham, particularly the one found in Genesis 22:17-18, as a foreshadowing of the universal spread of the Christian gospel of eternal salvation by faith in the atoning death of Jesus. According to this reasoning, the nations are blessed through Abraham's "seed" (i.e. Christ and his followers) in the sense that through them the offer of forgiveness for sins will extend over the whole of the earth. Abraham is thus given a taste of the gospel message that is to come.
Thesis: The aforementioned understanding of the promises to Abraham has little to nothing to do with the covenantal texts in the Genesis narrative. Rather, Abraham is promised fatherhood over a great and powerful nation, one through which God will bless and curse nations. Through Abraham's nation as a nation, not as a theological message or religious group, the nations will find blessing.
God makes promises to Abraham on a number of other occasions.
Genesis 12:2-3:
I will make of you a great nation, and I will bless you, and make your name great, so that you will be a blessing. I will bless those who bless you, and the one who curses you I will curse; and in you all the families of the earth shall be blessed.
Genesis 18:18
Abraham shall become a great and mighty nation, and all the nations of the earth shall be blessed in him
God makes identical or similar promises to Abraham's sons, Isaac, Jacob, and Judah. The blessing passed from Jacob to Judah is instructive:
Genesis 49:10, cf. 27:27-29
The scepter shall not depart from Judah, nor the ruler’s staff from between his feet, until tribute comes to him; and to him shall be the obedience of the nations.
The point I'd like to stress is that these promises and expectations await an earthly nation, one with real political clout, military strength, and prosperous land. This "great and powerful" nation will "possess the gates of its enemies" and receive the "obedience of the nations."
The idea that Abraham's offspring will bless the nations by means of a religious message about afterlife-salvation is absent. We are instead told that God will bless (materially) nations that bless Abraham's people and curse (materially) nations that curse Abraham's people. This blessing for some nations will come on account of their voluntary socio-political subordination to Abraham.