Albert Barnes Commentary Isaiah 19:21

"And Jehovah shall be known to Egypt, and the Egyptians shall know Jehovah in that day; yea, they shall worship with sacrifice and oblation, and shall vow a vow unto Jehovah, and shall perform it." — Isaiah 19:21 (ASV)

And the Lord will be known to Egypt - He will be worshipped and honored by the Jews who will live there, and by those who will be converted to their religion.

And the Egyptians will know the Lord - There can be no doubt that many of the Egyptians would be converted to the Jewish religion. This was the result in all countries where the Jews had a residence (compare the notes at Acts 2:9-11).

And will offer sacrifice - They will offer sacrifices to Yahweh. They would naturally go to Jerusalem as often as practicable and unite with the Jews there in the customary rites of their religion.

And oblations - The word מנחה minichāh, ‘oblation,’ denotes any offering that is not a “bloody” sacrifice—a thank-offering; an offering of incense, flour, grain, etc. (see the notes at Isaiah 1:13). The sense is that they would be true worshippers of God.

They will vow a vow ... - They will be sincere and true worshippers of God.

The large numbers of the Jews who lived there, the fact that many of them doubtless were sincere, the circumstances recorded (Acts 2:9–11) that Jews were in Jerusalem on the day of Pentecost, and the fact that the true religion was carried to Egypt and the Christian religion established there—all show how fully this prediction was fulfilled.