The answer given is, “Our first parents’ sin was eating the forbidden fruit.”
In our fallenness, we are likely to respond, “what’s the big deal about eating fruit?” Why would God kick our first parents out of the Garden for this?
We have to find the answer in the nature of God and why he created us in the first place.
Love always demands a choice. God did not create us as automatons who can only perform the tasks we are programmed to do. We are not simply a collection of Pavlovian responses. We are not merely “dancing to our DNA.”
We (as represented by our first parents) were given a choice and we chose badly.
If you look at the story in Genesis 3, you see that Satan asked a question that lead to a misunderstanding and misrepresentation of God’s character. We believed the lie that God does not have our well being in mind. Satan implied, and we believed, that God was withholding a good thing from us for no good reason.
Honesty forces us to admit that we still choose badly much of the time. We want our own way and struggle to see how obedience to God is for our benefit.
But isn’t that so like us? We know that the large dessert will move us farther away from our goal to maintain our optimum weight but we eat it anyway. We know that 20 minutes of exercise before work will go a long way toward keeping us healthy, but it is so much easier to check our social media feeds or grab 20 minutes of extra sleep.
We accept a short term pleasure at the cost of long term health.
Morally, we fare little better. We are serial idolaters, giving our best to things that draw us away from God. We want what we want when we want it and can find any number of excuses as to why it is the right course of action, even when the small voice inside us tells us we are off track.
Thankfully, there is good news. There is the Gospel (which means good news). We rebelled, but God had a plan to fix what was broken, even before we broke it.