Why did God not simply communicate this to them?
He did. Why do you think he hung back while the apostles went out to baptize? John was the
precursor. Jesus hadn't come to do the same thing John was doing. That's why John said, "I baptize you with water, but he will baptize you with fire and the Holy Ghost." If, at one time you believed that, I'd certainly like to know what made you stop.
The apostles had trouble letting go of their religious indoctrination. Jesus straight up told them, "The flesh counts for nothing; the words I speak to you are my spirit, and they are life." His teachings.
That is the living water with which we're meant to baptize people. It's not about the flesh. It's about interacting with the spirit via the teachings of Jesus in day to day life. I really don't get how you got your perspective so worked around again.
God had to order Peter to eat legally unclean food. Paul rebuked Peter for acting differently around Jews than he did with gentiles. Peter was still getting it wrong even after his time with Jesus. People were getting it wrong all over the place. What about circumcision.? What about Holy Days? What about feasts and festivals? Everyone was trying to understand this new way of thinking and there was a lot of confusion about it. So yeah, Paul carried on with water baptism because there's nothing morally or spiritually wrong with it. That's why Jesus let the apostles keep doing it, (at least for a while). It's just that people invariably mistake the ritual for personal goodness, even long after the fervor of the ritual has faded away.
As you yourself said, Paul came to realize that the baptism the people needed was not water. It was preaching Jesus. He was mistaken for some time. New information came to light which informed his decision making and he changed. He got a new perspective. It dawned on him that this was what Jesus meant by all that stuff about people cleaning the outside of their cup, while the inside is still full of filth.
What we need is a more common sense interpretation of these concepts. It's not the words that are important, but the
spirit behind them. That is the value Jesus brings; a spirit which is able to navigate every circumstance where the law can only fail. The spirit already has everything good from the law; it's just a better tool. God wants us to grow up, and part of that is learning to move away from these outward appearances of goodness and get back to simple obedience to Jesus' teachings.
That's the truth that sets us free. When you try to explain it with that two-kinds-of-salvation thing, the contradiction only becomes even more noticeable. i.e. "I's not necessary, but still, it's necessary". This happens when you're smart enough to realize that the water doesn't actually do anything so it makes no sense for it to be of spiritual importance, but tired enough to settle for an outward appearance that isn't inherently wrong. Jesus warned not to do our charity in public. He didn't warn not to get water baptized, so there is an inference, the same as all his other comments about the physical vs the spirit: let those who have ears, hear.
There's actually
a lot of other examples of Jesus promoting the spiritual vs the physical, even to the point of declaring that seeking the spirit is more important than even food and clothing. Remember, "Destroy this temple and I will rebuild it in three days"? He's clearly talking in a spiritual manner indicating his resurrection from the dead, but he's using language deliberately calculated to target the foremost physical obsession in the people's life at that point; the temple. He's talking spirit and all they can hear is physical.
It's all over the place, which I guess is why I'm carrying on (or caterwauling?) like this, writing all these paragraphs. I just can't believe you could be persuaded to go back to the physical. I mean, c;mon, look how Paul starts his letter to the Galatians in chapter three, rebuking them for becoming "bewitched" back into old testament works of the law. One of those old testament works is water baptism. It's common to assume it's a Christian thing because of John the Baptists' inclusion in the new testament books, but Jesus said that John was old testament. "The law and the prophets were until John,
since then the Kingdom of Heaven is Preached. " Yeah, that makes sense. Jesus was the guy with the message. John just warmed the people up for him.
In the new testament, you don't show your faithfulness to God by performing rituals which only
symbolize some inner desire to change. You do it by just getting busy doing the things Jesus said to do.
That is the demonstration of renewed cleanness. He straight up says it as plainly as he can in John 15, "Now you are clean
through my words". No ritual water necessary, just a desire to adhere to the words of Jesus.