“Then you shall say to Pharaoh, ‘Thus says Hashem: Yisrael is My first-born son” EXODUS 4:22 (The Israel Bible™)
Hashem (God) refers to the Children of Israel as His first-born son. A firstborn is not the only child, but yet his status is unique and therefore carries extra responsibility. Similarly, all nations of the world are Hashem’s children; anyone can form a meaningful relationship with Him. However, He chose the Jewish people as a “firstborn” to fulfill a unique role; to be His representatives to bring Godliness into this world. Similarly, He also chose the Land of Israel as the place where they are to fulfill that responsibility.