Why did G-d, the G-d of the whole Creation, chose to be the G-d of one nation?

This is the question that one must begin to understand in order to understand the Good News of the Kingdom of G-d.
These verses in Deuteronomy 32 and the verses in Zephaniah 3 and those elsewhere that reflect these verses in Deuteronomy, portray G-d as bringing a demonstration of mercy out of a demonstration of justice, a demonstration of His justified anger. This demonstration of anger and justice and forgiveness and mercy is all directed toward Adam corporately, toward Humanity as one creature of G-d. In order to set a stage for this demonstration, G-d set one family, one corporate segment of Humanity, apart in order to be able to demonstrate His purpose of mercy toward Adam through that family at the same time as His wrath was being displayed toward Adam as a whole, through His curse upon Adam in response to Adam’s rebellion. Insomuch as this family, whom God called in Abraham, Isaac and Jacob and named Israel, was a segment of Adam the wrath of G-d would be demonstrated in relation to them as well, but insomuch as they were set apart for the ultimate purpose of G-d, which was mercy and not strict justice, redemption and salvation would be demonstrated through them.