The statement found in verse 24, which the prior verses lead up to, is a pivotal, defining doctrine of the purpose of life. "What is this that thou hast said, that man should rise from the dead and be changed from this mortal to an immortal state, that the soul can never die... there was no possible chance that they should live forever?" (vs. 20-21)
One of the rulers of the people of Ammonihah referenced "that God placed cherubim and a flaming sword on the east of the garden of Eden, lest our first parents should enter and partake of the fruit of the tree of life, and live forever." He was without a correct understanding of the events that took place in the garden of Eden, a serious defect that kept them from understanding the plan of redemption.
Alma in the following verses goes on to explain how this action had to be taken to prevent Adam from partaking of the tree of life. He had already partaken of the tree of knowledge of good and evil (the "forbidden" fruit as it is traditionally referred to). But now, to prevent Adam from partaking of immortality prematurely, he had to be kept from the tree of life by angelic guard so that he would in fact be able to die, as God said he would. (See vs. 23)
And we see that death comes upon mankind, yea, the death which has been spoken of by Amulek, which is the temporal death; nevertheless there was a space granted unto man in which he might repent; therefore this life became a probationary state; a time to prepare to meet God; a time to prepare for that endless state which has been spoken of by us, which is after the resurrection of the dead. (vs. 24)The significance of this statement become more apparent when we begin to search for supporting verses throughout the scriptures. In reality, there only a few key references that point to this pivotal doctrine, most of which are found in the Book of Mormon or in the Doctrine and Covenants.