What was the sin Adam and Eve committed? Was it just eating a piece of fruit?

No. Something very serious. The fruit's just a symbol of what they did to God. If you remember the conversation the devil had with Eve, it was: God doesn't want you to eat the fruit because He knows when you do, you're going to become just like Him, a god, knowing good and evil. That's why He named the tree the tree of the knowledge of good and evil. Can you imagine? You, Eve, could be a god. And Adam too. You won't have to bear children. You won't have to till the garden. You'll be free from any work God wants you to do, and any purpose He has, you're not interested, because you're a god; you're going to make your own interests. And they said, yes, devil, you're right, and God lied to us, basically. So what did they do? They turned the devil into God, and they turned God into a devil that lies. There's no sin greater than that. As a matter of fact, if you look at the unforgivable sin, it's very similar to this one. In that scene in Mark 3, Jesus has already cured somebody right in front of the Pharisees, and instead of admitting that he couldn't be cured by a normal man, but only by a God-man, a divine being, they said He does these miraculous healings because He's under the power of Beelzebub, the prince of the demons. And the normal way to think of Jesus and His miracles is that He gets the power to do them from the Holy Spirit. Remember, Philippians 2:7 is the passage that tells us that Jesus Himself limited His God-man nature to man and laid His divinity on the shelf, so to speak, so that He would be dealing with us from the perspective of His human nature. There are some questions about that, but mostly that's the way the Gospels portray it: while Jesus is on earth as a man, He's getting His power to do miracles from the Holy Spirit. So Jesus turns to the Pharisees in Mark 3 and says, of all the sins that men sin, this one, blasphemy against the Holy Spirit, will not be forgiven. When you say Jesus is under the power of the devil to work miracles, you are blaspheming the Holy Spirit, because it's the Holy Spirit that gives Jesus the power to do the miracles, and you're saying He's the devil. And that sin will never be forgiven. But that's similar to what Adam and Eve did in the Garden of Eden. They basically blasphemed God, making Him the devil and saying He's a liar, because He doesn't want us to eat of the fruit, because we're going to become a god and push Him out of the way. And if you're a liar, you're the devil. And they made the devil look like God. Now, the Pharisees are in a time where Jesus has done so many good miracles, raising people from the dead, putting limbs back on people, feeding 5,000 with five loaves and two fish, that nobody has an excuse. They know instinctively that nobody can do these kinds of good works unless God were with him, and Jesus is claiming, God is not only with Me, I am God. They've had plenty of evidence that Jesus is different, but they're just not going to accept it, because they're in control of this religion. They're making money hand over fist. They control the lives of every single person who lives in Judea. They're living like kings, and they've learned how to deal with the Romans. And this Jesus is the only one that's come along ready to topple the tables and get them out of business quick. So they have to kill Him. Their situation is a lot different than Adam and Eve's. That's why Adam and Eve weren't told that such a sin, saying that God is the devil, is unforgivable. Not at that time, not in that environment. But with the Pharisees, yes, because they've reached the ultimate sin. And so Jesus goes pound for pound: you do that, your sin is not forgivable. You want to play that game? I'll play it with you.