Israel prepares to receive instruction from God and is given its vocation in the world.
Once again the context for these verses is being brought out of Egypt: "You have seen what I did to the Egyptians, and how I bore you on eagles' wings and brought you to myself" (19:4). A facile appropriation of this text leads to presumption. The logic might go like this: The whole world is God's, and we are God's special possession; therefore we have been designated to run the world. But the text moves in a different direction. God fought for Israel (14:14, 25), but Israel is not appointed as God's warriors. The assigned vocation is that of being a priestly kingdom and a holy nation. These designations open Israel to a vocation of service. Their set-apartness is for the sake of something other than themselves.
Christians will not be able to read this text without hearing 1 Peter 2:9. The latter does not counteract or contravene Exodus 19. Just as it was astounding and beyond imagination that God would choose the people without righteousness (Deuteronomy 9) and without power-a slave people (Deuteronomy 7)-so it is astounding that God calls people out of darkness into light. Part of the astonishment is that humans ought not seek to limit God's capacity to be astonishing. Being a priestly kingdom and a holy nation is not how Israel would be characterized based on the preceding murmuring episodes, but that is God's designation for them. When the Holy One later speaks from the cloud on the mountain in the midst of thunder and lightning, Israel will be terrified. The Holy One might break out in their midst and consume them, but to be a holy nation implies that the Holy One is in their midst. Israel will find this a hard tightrope to walk. In this respect the later details in the instructions for the tabernacle are not at all surprising.
5Now therefore, if you obey my voice and keep my covenant, you shall be my treasured possession out of all the peoples. Indeed, the whole earth is mine, 6but you shall be for me a priestly kingdom and a holy nation. These are the words that you shall speak to the Israelites.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org