The Lord told the Israelites that they must destroy the idolaters and their idols from the land of Canaan which they are going in to possess. They are not to move in and live among the heathen tribes of the promised land. They are to remove them and take their place, and when they do they must remove all traces of the false religious practices that have been going on there for many centuries. Why? So they will not be corrupted by them, "For you are a people holy to the Lord your God. The Lord your God has chosen you out of all the peoples on the face of the earth to be His people, to be His treasured possession." (Deuteronomy 7:6)
Why did the Lord choose Israel, out of all the peoples on the face of the earth at that time, to bless more than any other nation? One commentary I consulted phrased it like this and I think this is one of the best explanations out there: "He saw the unique potential in them to become a treasured people." There was something unique about Israel, for no other people at that time---no nation as a whole---served the Lord. Most nations had rejected Him entirely, though they knew of Him and though their ancestors may once have worshiped Him. Scattered among the nations were those who still believed in Him, for we've found priests and prophets of the Lord in the Old Testament who were not Israelites. But the vast majority of the people of the world cared nothing for the Lord and had abandoned Him in favor of idolatry. Israel alone, as an entire race of people, remained faithful to the Lord.
The Israelites stood for the Lord and stood out from the crowd. They shone like beacons in a dark and fallen world. They were the only nation capable of lighting the way to salvation because they were the only nation who served God. The Lord promised Abraham (who rejected the false gods of his fathers and answered the call of the one true God and set out in faith for a land he'd never seen) that, "Through your offspring all nations on earth will be blessed, because you have obeyed Me." (Genesis 22:18) The Lord is keeping His promise to bless Abraham's descendants and to bless all nations through the offspring of Abraham, for it is into the nation of Israel the Lord will send His Son, a descendant of Abraham. It is through His Son that all nations will be blessed.
The Lord didn't choose Israel because she was the mightiest nation on earth. She wasn't. He didn't choose Israel because her army was the most powerful. It wasn't. He chose Israel because He loved Israel, and although it's true He loved everyone on the face of the earth, only Israel reciprocated His love. So imperfectly she reciprocated His love, but isn't that true of us all? Yet He kept His promise to Abraham to give the land to his descendants and to make a great nation of them. "The Lord did not set His affection on you and choose you because you were more numerous than other peoples, for you were the fewest of all peoples. But it was because the Lord loved you and kept the oath He swore to your ancestors that He brought you out with a mighty hand and redeemed you from the land of slavery, from the power of Pharaoh king of Egypt. Know therefore that the Lord your God is God; He is the faithful God, keeping His covenant of love to a thousand generations of those who love Him and keep His commandments." (Deuteronomy 7:7-9)
We see that, aside from His love for Israel and Israel's love for Him, the Lord blessed Israel above all other people for Abraham's sake. Abraham had been dead for centuries by the time the Israelites entered the promised land but for Abraham's sake the Lord was still keeping His word. The Lord does not break His word even when man breaks his. We won't find one hundred percent of Israel being faithful to the Lord all the time but the Lord will remain faithful. The Lord won't refuse the promised land to Israel. The Lord won't go back on His oath to make Israel into a great nation. The Lord won't decide not to send the Redeemer to Israel---the One through whom all nations will be blessed. The Lord said He would do all these things for the descendants of Abraham and He will do all these things.
If the faith of one man is still paying off many centuries later for his descendants, we cannot truthfully say, "I'm just one person. What can I do?" Abraham was one man who believed on the Lord in Ur which was filled with people who worshiped false Gods, but because he was willing to trust the Lord wholeheartedly and follow Him wherever He led, the Lord continues to bless his descendants. The faith of one man accomplished much. How might your faith influence others? How might your love of the Lord and your obedience to Him keep paying off for generations to come? What might He do for your grandchildren or great-grandchildren for your sake?