To understand the justice of God’s punishment, Alma finds that he must return to the way that opposition was set in motion for all of humankind. He returns to the Garden of Eden, a story that Corianton surely knows, and therefore Alma gives only a brief outline. The part he chooses to tell is that after partaking of the fruit of the tree of knowledge of good and evil, Adam and Eve had to be prevented from partaking of the fruit of the tree of life. This is the tree of life in the Garden, which has a slightly different imagery from the one in Lehi’s dream.
The elaboration of the meaning will come in verses 5 and 6, but Alma sets up that meaning by suggesting that by forestalling eternal life, a time was created in which humankind could learn and, through repentance, still achieve that eternal life.