What does Isaiah 19:21 mean?
Isaiah continues by describing Egypt's transformation. They are the object of the Lord's judgment in Isaiah's near future, and then a people in genuine relationship with the Lord in the distant future. Not only will Egypt be rescued from her oppressors by the Lord, but they will also come to know Him as their Lord and the source of their security (Isaiah 19:18–20).Egypt had "known" the God of the Israelites before, but only in the shallowest or most negative sense of that word. They understood that Israel's God was the one sending plagues and suffering upon their people during the time of Exodus (Exodus 7:5; 9:28). Now they will know the Lord in the most positive context by being in a worship relationship with Him (Psalm 100:3). Egypt's worship will include sacrifices and offerings to the Lord. This worship will also include making and keeping vows to God.
In Israel's worship, vows were made and performed as an expression of thanks to the Lord (Psalm 22:26). These vows were "paid off" when the Lord answered prayer in a specific way (1 Samuel 1:11). The fact that the Egyptians will also take part in such worship practices shows just how deeply they will participate in the worship of Israel's God.