Baptism and Eschatology and 70 AD
In 1Corinthians 10, Paul compared the post-Exodus Israelites to the first century Christians. His point was to warn of the danger of falling away, and to warn that falling away was a present reality, just as it was for the Israelites post-Exodus.
He pointed out that the Israelites had been led out of Egypt and then "baptized into Moses" by the cloud and the sea.
Whenever we see the phrase "baptized into" in the New Testament, it refers to a process of pledging oneself. John "baptized into" repentance; that is, the people he was baptizing were pledging themselves to repentance. He also "baptized into" forgiveness; the people were being baptized to identify that they had been forgiven, or that they were seeking forgiveness.
Jesus talks about being "baptized into" the Father, the Son, and the holy Spirit. Paul talks about being "baptized into" the death of Christ. Each of these refer to identifying or pledging oneself to a certain cause or individual.
When Peter speaks on Pentecost, he does the same thing. "Change your minds, and be BAPTIZED in the name of Jesus Christ INTO the forgiveness of your sins." These people were being "baptized into" forgiveness (the same as with John) identifying themselves with Jesus and the forgiveness he brought.
So when Paul speaks in 1Corinthians 10 about being "baptized into" Moses, he is not focusing on the water aspects of passing through the Red Sea. He is saying that the Israelites had pledged themselves to Moses by following him through the Red Sea, and by following the cloud of God's glory. By doing so, they had left their old lives behind, utterly and completely.
So the sequence is as follows.
The Passover occured in an event symbolizing the purchase of their lives. Moses then led the people out of Egypt in the beginning of the Exodus. When they got to the Red Sea, the people followed Moses through it, fully cementing their identification with him, and their rejection of their old lives. They then began a period of 40 years of wilderness wandering.
Paul's comparison of the First Century Christians to the ancient Israelites is rife with symbolism.
They too had witnessed a Passover, with Jesus the Messiah as the sacrificial lamb purchasing their lives. They too had been baptized to identify themselves with Jesus and his new movement, and in rejection of their former lives. They too had entered a 40 year period of persecution and struggle. Paul warns that they too could fall back into their old ways, and end up being destroyed as the ancient Israelites were. But if they persevered, they too would eventually enter into the Promised Land.
In making this comparison, Paul revealed a lot about First Century baptism. It was a pledge and identification with Jesus. It both symbolically and actually closed the doors on their old lives, because identifying with Jesus meant rejection by your friends, family, and the religious establishment. Baptism meant entering into the 40 years of persecution and rejection from those you had known.
Only one generation of Israelites were "baptized in the cloud and the sea". Future generations made their pledges to God in other ways; but this one historic action had ramifications for the entire future of the nation.
In a similar way, first century Christians who publicly identified with Jesus through baptism established themselves as a counter-cultural force whose actions and story would reverberate down through the ages. The way God dealt with this publicly identified group would send a message to the rest of the world.
When God vindicated these Christians in 70CE, their public identification with Jesus changed from a mark of persecution to a mark of triumph, and sent a clear message about God's nature ringing down the halls of history.
Without the public identification of that chosen generation, the victory of Jesus would have been vague and indistinct. But by choosing to publicly identify and suffer with Jesus, that generation established an object lesson for all time, and achieved a stunning vindication.
That was the purpose of baptism.