Why did God let evil in?

One evening during a prayer meeting at Cristo Restaurador, we divided up into groups of 2 or 3 and after we prayed, one of the members of our church asked me this question. He was asking why God even let the forbidden tree in the garden or the devil enter in to tempt Adam and Eve. I understand the question to imply another assumption/question, "Wouldn't it have been better if we never had the chance to fall into sin?"

In Genesis 1-3, we read about God creating the heavens and the earth and it all being good and very good. We read about God establishing the one requirement with Adam. God said he could eat from all the trees of the garden except from the one tree of the knowledge of good and evil. He said that on the day that he eats of it, he will surely die. Sadly, we read on about the serpent entering and challenging this command and Adam and Eve eating the fruit, of their eyes being opened and how they knew that they were naked. Throughout history they and then their descendants one after another all died. But, God said that on the day that they eat they would surely die. Satan used this on Eve when he said they would not really die, only that their eyes would be opened. Their eyes were opened. They didn’t fall down dead like Ananias and Sapphira. Was Satan right? Where is that death that was to happen on the same day? It is the death of sin. Immediately, they knew they were naked. Shame entered. Their relationship with God was broken. Sin contaminated every thought and action from that point on. Their children were born sinners and broken in sin. (Romans 5:12-21)

Why did God let this happen? Why did he even put that tree there for them to be able to eat? Why did he let the devil in the garden?

I am reminded of a seminary class on Hebrew looking at this Genesis passage and the professor went to Jesus’ prayer in John 17: 20-26 in response to this very death of Adam and Eve. Jesus prays to his Father that we would have a unity with each other and even more with himself and the Father. Jesus brings a whole new level of union with God that Adam and Eve did not experience. So this leads us to believe that God allowed evil to enter in order to send his Son to redeem us and give us something even better than what Adam and Eve had in the garden.
Even more in John 12:20-26. Jesus curiously responds to the news that several Greeks were looking to see him, that it is his hour to be glorified and then introduces the idea of a seed needing to die in order to bear fruit. He was talking about his death, it is curious that he used the illustration of a seed needing to fall into the earth and die. He goes on to challenge us that anyone who wants to serve him must follow him and surrender his life. He says that if we love our life we will lose it, but if we hate our life in this world we will keep it for eternal life. Paul expounds on this same idea of planting a seed in order for the new plant to grow and bear fruit.
1 Corinthians 15:35-58. Paul is talking about our hope in the resurrection. He touches on the same idea of a seed needing to die so that the plant can then grow. He further explains (and connects this idea of why God allowed death to enter) that the plant that grows is much fuller and better than the bare seed that was planted. In other words, the plant life is better than the seed life. What Adam and Eve had was good. But it was nothing in comparison with what Jesus was talking about in his prayer. They walked with God but they did not have this kind of union with Him. The glory of the resurrection body is so much better than the merely physical body just as a plant is more glorious than the seed.  Romans 8:18 also says that the sufferings of this life do not compare with the coming glory to be revealed to us. This seems to mean that death and resurrection is the means that God provided for us to enter into His glory. It wouldn’t make sense to have resurrection without there first being death. There is no sense of having redemption with out first having a fall. So God allowed death and evil to enter as a part of his plan to unite us with him in glory beyond comparison. He purposefully put the tree in the garden and allowed the devil in to tempt them so that they could fall and die and then be brought into glory through the resurrection of Jesus Christ. Just as a gardener will purposefully place a seed in the ground and bury it, God allows us to suffer and die as a part of the process that He designed to give us something so much better than before. The plant that grows up out of the ground is so much more glorious than the bare seed. In the same way that glory that we participate in is so much better than the carnal life of sin and death.  Jesus conquered death when he was crucified, died, buried and then rose again in the new creation of the resurrection and now gives that new life to us through faith in Him. He prayed for us, that the Father would give us this unity with Him through His death and resurrection. So now, we have an amazing unity with the triune God as a result of God allowing Satan in to the garden to tempt our first parents. Satan thought he was destroying something good but he ended up being a critical piece in making something good turn into the most amazing reality in all of creation.