Scripture shows us that God chose Abraham and his descendants because of the faith he displayed in God. This faith makes Abraham and all who descend from him regarding displaying such faith particular and chosen. Ultimately all will be given the opportunity to live in faith, but the covenant with Abraham makes Israel the chosen nation.
And there is the interesting case of Melchizedek. But I always enjoy the quote from Fiddler on the Roof, when Tevye says, “Lord, I know we are Your chosen people…But, couldn’t You choose someone else sometime?”
Indeed - I too enjoy that treatment of being chosen.
I believe original sin is something that evolved. Animals and early hominids are/were incapable of sinning because they had not “eaten from the tree of knowledge of good and evil”. God in one sense did not want us to obtain knowledge because we would have the ability to sin (but He knew we would get there). The more knowledge we get, the higher capacity to sin, and the worse the natural consequences could be. I find it entertaining that we associate the fruit of tree of the knowledge of good and evil as an apple. Sir Issac Newton was inspired by a falling apple to wounder about gravity and gave rise to modern physics. This inspired Einstein to develop the Theory of Relativity which later gave us nuclear weapons. Nuclear weapons gives us the capacity to help bring about Armageddon. There is a point in our development where God will see us as knowing right from wrong and having the ability to sin. We will also have the chance at knowing and accepting Jesus giving us access to the Tree of Life again (being with God in Heaven). After we sin we cannot eat from it unless we accept Him. This comforts me in knowing that when a young child dies they will be in Heaven with God, because they did not get to eat of the tree of knowledge yet. Notice once man knew he was naked he is capable of sinning. Little kids love running around naked with no shame. Once they have shame they need to know Jesus and accepted Him.
I would consider God’s interaction with Abraham, including the appearance of Melchizedek, to be part or the process of carrying out his previous choice of the Jews. Abraham’s faith became a message for us.