Under the gospel of the kingdom preached to Israel only, water baptism is a requirement for salvation. Israel as a nation was commanded to accept Jesus as their Messiah and be ushered into their promised kingdom where Jesus will rule over them from David's throne in Jerusalem.
Then Israel will become a kingdom of priest, in order to be a priest, every Jew MUST be baptized (
Exodus 19:4-6)
Once that happen, then every Jew will be responsible to spread the gospel to every Gentile nation under the Earth. There are a number of OT passages that talk about this prophetic timetable, the most beautiful to me is found in
Zechariah 8
3 Thus saith the Lord; I am returned unto Zion, and will dwell in the midst of Jerusalem: and Jerusalem shall be called a city of truth; and the mountain of the Lord of hosts the holy mountain.
7 Thus saith the Lord of hosts; Behold, I will save my people from the east country, and from the west country;
8 And I will bring them, and they shall dwell in the midst of Jerusalem: and they shall be my people, and I will be their God, in truth and in righteousness.
20 Thus saith the Lord of hosts; It shall yet come to pass, that there shall come people, and the inhabitants of many cities:
21 And the inhabitants of one city shall go to another, saying, Let us go speedily to pray before the Lord, and to seek the Lord of hosts: I will go also.
22 Yea, many people and strong nations shall come to seek the Lord of hosts in Jerusalem, and to pray before the Lord.
23 Thus saith the Lord of hosts; In those days it shall come to pass, that ten men shall take hold out of all languages of the nations, even shall take hold of the skirt of him that is a Jew, saying, We will go with you: for we have heard that God is with you.
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But as we can all see by Acts 7, Israel has rejected their King, they even stoned Stephen who was full of the Holy Spirit. When that happened, Jesus saved Saul and revealed to him a mystery, that because of Israel's rejection, all the Gentiles can now be saved without them (
Romans 9 to
Romans 11).
How does one get saved now? That is thru 1 Cor 15:1-4. Water baptism is no longer required for anything under this mystery grace dispensation.