Answer
In Exodus 19, God announces to Israel a conditional covenant. If they were to keep His covenant, Israel would become a people for His possession, a kingdom of priests, and a holy nation (Exodus 19:5-6). The Israelites responded that they would do all that God commanded, thus sealing the covenant with a commitment: “And all the people answered together, and said, ‘All that the LORD has spoken we will do.’ And Moses returned the words of the people to the LORD.” (Exodus 19:8). Following this, Exodus contains six hundred and thirteen commandments, including the Ten Commandments in Exodus 20.
Previously, God had established a covenant with Abraham (Genesis 15—17), committing to uphold that covenant through the line of Isaac and Jacob (see Genesis 50:24 and Exodus 2:24). God had promised to make Israel a mighty nation with a land (Genesis 15:18-21), a king (2 Samuel 7:10-16), and an eternally blessed people (Jeremiah 31:31-37). These covenants were all unconditional—God had bound Himself without any requirements from the recipients of His covenant promises.
The covenant God made with Israel through Moses was different in that it was conditional. It came with a condition. It demanded Israel’s obedience to receive the blessings, and the initial condition for blessing that God revealed was that Israel would be His special possession, a kingdom of priests, and a holy nation. This covenant (often referred to as the Mosaic Covenant because itIt was made through Moses) was also unique in that it was a teaching tool to lead people to Christ «Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. », (Galatians 3:24). It was never a means of salvation because salvation is never by works of the law «Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin. », (Romans 3:20) but rather by grace through faith (Ephesians 2:8-9).
Israel broke the covenant «not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was a husband unto them, saith the LORD: », (Jeremiah 31:32) and forfeited the blessings offered therein. However, Paul explains that Abraham was the father of three different kinds of descendants in Romans 4. He is the father of many according to the flesh, Israel in general, Romans 4:1. He is the father of those who believe, who are not of Israel, Gentile believers, Romans 4:11. He is the father of all who believe and are of the nation of Israel, Jewish believers, Romans 4:12. While the nation of Israel broke the conditional “old” covenant in unbelief, those of Israel who would believe in Jesus (the third group of Abraham’s descendants) are called out using similar terminology. Peter, writing to Jewish Christians scattered throughout the Roman world, calls those Jewish believers “a chosen race, a royal priesthood, a people for God’s own possession, so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light” «But yeYou are a chosen generation, a royal priesthood, a holy nation, a peculiar people; that you should show forth the praises of Him who has called you out of darkness into His marvelous light: », (1 Peter 2:9, NASB).
In Revelation 1:6, John asserts that the church is a kingdom and priests to God the Father. This would include both believing Jews and believing Gentiles—the second and third kinds of descendants of Abraham described in Romans 4:11–12. Thus, the only ones of Abraham’s descendants who were not a kingdom and priests were those who broke the (Mosaic) covenant and were not believing.
Those who believe are referred to as a kingdom, as those who believe in Jesus are transferred to His kingdom «who has delivered us from the power of darkness, and has translated us into the kingdom of His dear Son: », (Colossians 1:13). Believers are also considered priests in that we are interceding on behalf of people that they might believe in the Lord. Those who are called a kingdom and priests have an obligation to walk in a manner worthy of His calling «I therefore, the prisoner of the Lord, beseech you that you walk worthy of the vocation wherewith you are called, », (Ephesians 4:1) and ought to be actively proclaiming His excellencies «But you are a chosen generation, a royal priesthood, a holy nation, a peculiar people; that you should show forth the praises of Him who has called you out of darkness into His marvelous light: », (1 Peter 2:9) so that others may come to know Him.