What is the nation of Israel supposed to become according to God, if they obey?
God says the nation of Israel is to become a kingdom of priests. If they are obedient, they are to be a nation of priests. A priest is a go-between between man and God, so Israel was to serve in that role.
Les explains from verse 6 that the nation of Israel, God says, is to be a kingdom of priests. He then asks what a priest is and answers that a priest is a go-between between man and God. So if Israel is obedient, they are to become a nation of priests. The surrounding context also notes that Grace Age believers are members of the kingdom, whose location is in heaven, and that the kingdom is coming back to the earth.