I was studying Genesis chapter 2 in order to teach elementary age children about God's creation of Adam and Eve, and I became captivated by the trees at the center of the garden (verse 9). The first is "the Tree of Life", and the second is "the Tree of the Knowledge of Good and Evil." Theologians love to argue about God's reason for either tree being there. The tree of Life seems almost an afterthought, and God does not seem to draw attention to it at all until He takes away man's access to it.
I don't know why I never saw this before, but it seems to me that God was presenting Adam with a choice. Not just the choice to eat or not eat of the forbidden tree -- rather, WHICH tree would he choose? of the tree of Knowledge, God said, "you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.” Gen 2:17 This tree could thus be called the tree of Death, making the choice between the tree of Life and the tree of Death.
We know the results of eating of the tree of Death because that's what Adam did, but what if he had chosen differently? What if, instead of falling to temptation, he ate of the tree of life? At a guess, the tree of Knowledge/Death would have been taken away and humanity would have had a very different history -- one without sin, one walked with God.
Looked at this way, other scriptures stand out as connected to this original choice. Deuteronomy 30:15-20 for example also sets before the Israelites (and, I believe, the reader) to choose "Life, blessing, and prosperity"; or "Death and destruction". As obvious as the choice would seem, we humans tend to choose the latter.
Ever since Jesus gave His life on the cross and rose again, each man and woman has that same choice: Life (through Christ) or Death (through man and the world). Unfortunately, because Adam chose Death, our natural tendency is to do the same. God must change our hearts and give us a new nature to choose Life.
Why then did God not make the choice clear? Why did He call it the "Tree of Knowledge" etc.? I think if He had called it the tree of Death and not attached any apparent value to it, Adam would not have had a real choice. For example, if I plant a tree that has poisonous fruit that smell and taste terrible (or at least are unappealing), then I tell my children not to eat of it, they are unlikely to be tempted by it. However, if the tree's fruit seem to have some benefit and look and even smell good, then when I tell my children not to eat it they must choose: obey their father and trust his good will or disobey and find out for themselves. Adam decided to disobey, and we have all found out for ourselves what good and evil are, and what death feels like.
And what of the serpent? Why did God allow him into the garden? His only goal seemed to be to lead man astray. Again, I think he was a part of the test. As the tempter, trying to draw man away from God, he was (is?) something of a counterbalance to God. In the end, there is only one way to eternal joy, but God, desiring our love, gave Adam the choice of whether to give it.