There is a significant group of Christians who either believe in:
- the necessity of water baptism to get saved, or
- the necessity immediately after you are saved, otherwise you may not be saved since you are disobeying a direct commandment from the risen Lord, which to me amounts to pretty much the same as the first belief.
Verses like Mark 16:16 and Acts 2:38 feature heavily in both beliefs.
The dispensationalist view towards water baptism is clear: Jesus was on Earth to fulfill the promise of a kingdom to the Jews, preaching the
Gospel of the Kingdom (Matthew 4:17).
Under that Kingdom, all Jews are supposed to accept Jesus as their King, and then be priests that will spread the message of the King to every nation.
This was prophesied in many OT prophets, some examples would be
Zechariah 8 explains
3 “Thus says the Lord: ‘I will return to Zion, And dwell in the midst of Jerusalem. Jerusalem shall be called the City of Truth, The Mountain of the Lord of hosts, The Holy Mountain.’
Later on, in the chapter in Zechariah 8:20-23 New Living Translation (NLT)
20 “This is what the Lord of Heaven’s Armies says:
People from nations and cities around the world will travel to Jerusalem. 21 The people of one city will say to the people of another, ‘Come with us to Jerusalem to ask the Lord to bless us. Let’s worship the Lord of Heaven’s Armies. I’m determined to go.’ 22 Many peoples and powerful nations will come to Jerusalem to seek the Lord of Heaven’s Armies and to ask for his blessing. 23 “This is what the Lord of Heaven’s Armies says:
In those days ten men from different nations and languages of the world will clutch at the sleeve of one Jew. And they will say, ‘Please let us walk with you, for we have heard that God is with you.’”
A similar prophecy was given in Micah Chapter 4 (NLT)
In the last days, the mountain of the Lord’s house will be the highest of all—the most important place on earth. It will be raised above the other hills, and
people from all over the world will stream there to worship.
2
People from many nations will come and say, “Come, let us go up to the mountain of the Lord, to the house of Jacob’s God. There he will teach us his ways, and we will walk in his paths.” For the Lord’s teaching will go out from Zion; his word will go out from Jerusalem.
Finally, the same prophecy was given in the book of Isaiah chapter 2:2
In the last days, the mountain of the Lord’s house will be the highest of all—the most important place on earth.
It will be raised above the other hills, and people from all over the world will stream there to worship.
So, based on these 3 accounts,
the timeline was supposed to be Jews are to be saved first, once they accept Jesus as the Messiah, then the Kingdom will be established in Jerusalem and the Jews will then be priests spreading that blessings to all the Gentiles.
So in order for Jews to fulfill the role of priests, all of them must be water baptized. This was not a requirement before John and Jesus came on Earth, but because both of them came in preaching the Gospel of the Kingdom, water baptism became required as part of the salvation process.