A lot depends on your theology. Mine tends towards the latter interpretation, that the tree was one of knowledge. Adam and Eve had to partake of the fruit in order for Christ's Atonement to effect mankind's progression. In other words, the fall of man was pre-planned with the intention of exalting man to levels impossible to attain for Adam and Eve. Adam and Eve both had the capability of living forever, but they would have had their ability to progress severely retarded. This is a doctrine shared by both Mormonism and the Eastern Orthodox, both which believe man's fall and atonement were engineered to advance man to a potential of deification; in other words, for God's children to be able to become as God is.
Man first had to attain a knowledge of good and evil, which was brought about by eating the "forbidden" fruit.
Technically, however, the fruit was not forbidden. God simply said, "You shall not eat of it, lest you die." Or, originally, "But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die." They were told they could freely eat of any tree, but God told them not to partake of the forbidden fruit because it would introduce death into the world, and He could not be seen as actively advocating it. It had to be man's choice. At the same time, it was necessary and God had already set up the plan by which man could be redeemed. In other words, Jehovah had already been chosen and ordained to bring about man's redemption and thus becoming co-heirs with Christ of everything that the Father had. It's also important to understand that Satan did not lie to man. He was right in saying that man would not die...immediately, and he was accurate in saying they would become as the Gods [Elohim], knowing good from evil. But he lied in making them think they would not become subject to death.
When they partook of the fruit, they immediately had their eyes opened to good and evil and began to understand they had been deceived. The Father, however, could not overlook the fact that man had sinned, and, according to the dictates of Justice, He had to appoint an intercessor, an advocate and redeemer. This was Jehovah, who later was born as Christ. It's difficult to read the "old" testament and not see the same names and titles of Jesus being applied to Jehovah (Savior, Redeemer, King of Kings, Lord of Lords; First and Last; Shepherd, etc.). In Psalms 110:1-2, David writes, "The LORD saith unto my Lord, Sit thou at my right hand until I make thine enemies thy footstool." But who was David's Lord by Jehovah? And if Jehovah was the second Lord, who was the first? According to the new testament writers, the first Lord was the Father.
Once man's (and Earth's) redemption is complete, Jehovah will present the glorified Earth back to the Father and his work will be completely fulfilled. And even though the Atonement is complete, we're still to ask the Father for the things we need in the name of Christ. Evangelicals don't understand the distinction because I've heard them pray to Jesus and then say, "We ask these things in thy name."
