The OT portion of today's reading is the story of the exodus, the single most important event in Israel's history, and the paradigm for all future deliverances God performed for the nation. It is a story of severe justice, as God struck down the firstborn of Egypt as a recompense for the way Egypt brutalized His firstborn son, Israel (see Ex. 4:22-23). Pharaoh ordered the murder of Israel's sons; God executed Pharaoh's firstborn son. If the final plague of the death of the firstborn seems harsh, it is only because the crime to which it was a response was so harsh.
Most of the OT revolves around historical events which pitted Israel against Gentiles, whether smaller states on the same level as Israel (like the Philistines), or major world empires that spanned the far reaches of the ancient world (like Egypt, Babylon or Persia). By the time of the NT, the greatest pagan empire of all imposed Pax Romana with brutal efficiency, and Israel seethed with resentment of the tight grip of Rome.
Who would have ever expected in the light of this animosity that God's plan to save Israel also included salvation for the nations? It is true that God promised Abraham that his family would bless all families of the earth, but most Jews expected this could only happen if the nations of the earth became one with Israel on the basis of the Law. If the Gentiles would embrace the Torah, obey the covenant of circumcision, and cast their lot with Israel, they could be blessed.
But as Paul wrote in today's NT reading, God's actual plan to bless the families of the earth was much different, and only in Christ was this mystery revealed: "This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel" (Eph. 3:6). Here is what Paul says God had always intended to do, but only disclosed in the gospel:
1. Gentiles would become fellow heirs with Israel, rather than being second-class members of the covenant.
2. This would happen on the basis of the gospel rather than the Law.
3. The Messiah would be the one who made this possible.
How unimaginable it must have been for any Israelite watching the waters of the Red Sea devour their pagan enemies that centuries later, Egyptians (or Assyrians or Romans) could be part of the same family, that the Law would no longer identify God's true people, and that all of this would happen because of the work of the Messiah.
God works in a mysterious way
His wonders to perform;
He plants His footsteps in the sea
And rides upon the storm.