Which baptism is the question? The word "water" is not found in Mark 16:16.
Some of us see water every time they see the word "baptize" in scripture.
What do we find below in scripture?
Luk 3:16 John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:
Act 11:15 And as I began to speak, the Holy Ghost fell on them, as on us at the beginning.
Act 11:16 Then remembered I the word of the Lord, how that he said, John indeed baptized with water; but ye shall be baptized with the Holy Ghost.
Rom 8:9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
Eph 1:13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
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Water baptism has been required for Israel since John the Baptist came into the scene.
You can understand the apostle John's specific emphasis on being born again, as inextricably linked to Israel's relationship with God.
Their father, Abraham, was barren. Naturally, at his ripe old age, he could no longer have children.
God had to supernaturally intervene in his life to enable him to give birth to Issac.
Thru Issac, thru Jacob, thru his 12 sons, the nation literally came into existence.
Exodus 4:22 had a very insightful verse about this
And thou shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn:
God considered Israel his first born son. They literally became a nation when God supernaturally rescued them from Egypt, separating the waters in the ocean until all of them literally cross over the water in dry land.
But as we all know, Israel broke their covenant of Law that was given at Mount Sinai. They killed or ignored all the prophets that God repeatedly sent to them when they were separated into 2 kingdoms and went into captivity under Babylon, and then under Persia.
Jesus used the parable of the tenants, one of my favorite parables to understand his first coming on Earth to Israel, in all 3 synoptic gospels. (Matthew 21:33-46; Mark 12:1-12; Luke 20:9-19) to illustrate this.
When Jesus and the 12 were preaching from Matt-John, they need to repent of rejecting God their Father in the OT, and believe in his Son is their promised King and Messiah, as foretold by their prophets.
God had mercy on them even when they killed his prophets (e.g. 2 Chronicles 24:20-22; Jeremiah 26:20-30; cf. Luke 13:34; Acts 7:52)., and is now sending his very own Son to Israel, to do a final persuasion to Israel to repent and believe in him.
If they do, as Jesus told Nicodemus in John 3, Israel will be born again.
But you are correct, water baptism is not required for salvation for us now in the Body of Christ, we are saved by following 1 Cor 15:1-4. As I have already stated to you, Paul never mention to us the same necessity of being born again.