Isaiah 19:21 English Standard Version (ESV) And the Lord will make himself known to the Egyptians, and the Egyptians will know the Lord in that day and worship with sacrifice and offering, and they will make vows to the Lord and perform them.
Before we begin here let us clear up what might become errors in thinking.
First we are talking about the Lord. Not Father God or God Almighty or any other title that implies an identity of the Trinity that is not the Lord alone.
Secondly Egypt was visited by Moses and Israel was set free before this word of prophecy was given. Egyptians here is a representative image of peoples that worship false gods. That includes those sinners who do not believe in gods of any kind. They become gods unto themselves.
Now let us deal with the Lord making Himself known to sinners. This begins with an understanding of when Jesus became the Lord.
Acts 2:36 English Standard Version (ESV) Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified.
Jesus did not become the Lord or the Messiah until after His natural death. When He rose from the grave became the Messiah. Ascension placed Him on the Right Hand Throne of God and in that position became the Lord.
Jesus was the Son of God sent into the world to save sinners. John 3:17
The Messiah is defined by the International Standard Bible Encyclopaedia here. The term is used in the Old Testament of kings and priests, who were consecrated to office by the ceremony of anointing. It is applied to the priest only as an adjective-“the anointed priest”.
Hebrews 6:20 English Standard Version (ESV) where Jesus has gone as a forerunner on our behalf, having become a high priest forever after the order of Melchizedek.
A forerunner is an example for others. We who serve the Lord become priests after the order of Melchizedek. That is not a worldly title, it is a spiritual one.
Now we have to decide who we know, Jesus, the Messiah, or the Lord.