What was Israel to become if they were obedient to God?
If Israel was obedient, they were to become a nation of priests—a kingdom of priests—functioning as a go-between between man and God. This was God's stated calling for the nation, conditional on their obedience, and the context presents priests as intermediaries.
Les explains that God said the nation of Israel is to be a kingdom of priests. He asks what a priest is, then defines a priest as a go-between between man and God. So when the text asks what Israel would become if obedient, the answer is a nation of priests. This calling relates to Israel's role as intermediary, and in the broader context Les notes the kingdom is coming back to the earth, while Grace Age believers are members of the kingdom now with its location in heaven. He also references Acts 2:38, where Peter tells the house of Israel to repent and be baptized.