Moses 4

As the Lord would remaint truthful to His conditions, He tells us -

"So I drove out the man, and I placed at the east of the Garden of Eden, cherubim and a flaming sword, which turned every way to keep the way of the tree of life." (Pearl of Great Price | Moses 4:31)

We receive more insight as Antionah asked -

"What does the scripture mean, which saith 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? And thus we see that there was no possible chance that they should live forever." (Book of Mormon | Alma 12:21)

Alma answers -

"...if it were possible that our first parents could have gone forth and partaken of the tree of life they would have been forever miserable, having no preparatory state; and thus the plan of redemption would have been frustrated, and the word of God would have been void, taking none effect." (Book of Mormon | Alma 12:26)

Alms tells us that it was not possible that Adam and Eve could eat of the Tree of Life. Later to his son, Alma says -

"...after the Lord God sent our first parents forth from the garden of Eden, to till the ground, from whence they were taken—yea, he drew out the man, and he placed at the east end of the garden of Eden, cherubim, and a flaming sword which turned every way, to keep the tree of life—
3 Now, we see that the man had become as God, knowing good and evil; and lest he should put forth his hand, and take also of the tree of life, and eat and live forever, the Lord God placed cherubim and the flaming sword, that he should not partake of the fruit— (Book of Mormon | Alma 42:2 - 3)

But Alma follows with is the important part -

"...thus we see, that there was a time granted unto man to repent, yea, a probationary time, a time to repent and serve God.
5 For behold, if Adam had put forth his hand immediately, and partaken of the tree of life, he would have lived forever, according to the word of God, having no space for repentance; yea, and also the word of God would have been void, and the great plan of salvation would have been frustrated." (Book of Mormon | Alma 42:4 - 5)

What I think may be overlooked in these versus is the Cheribum that was placed to "keep" the way to the Tree of Life. Keep does not necessarily mean that one cannot get to the Tree of Life. Keep suggest maintaining control of the way.

It was Amulek that made it clear of the Savior that -

"...he shall not save his people in their sins." (Book of Mormon | Alma 11:36)

The Tree of Life is symbolic of the Savior and His redeeming love or grace. A time was given so that man could repent and become free of sin. The Lord said to Adam and Eve -

"...thou shalt repent and call upon God in the name of the Son forevermore." (Pearl of Great Price | Moses 5:8)

As we ponder Lehi's vision of the Tree of Life, which I believe is the same symbolic tree, he saw but one way, one gate, and a strait path or time that led to the tree. Then it only makes sense that one can reach the tree and partake of His fruit by following the Lord's way. Nephi also saw that all other ways were kept as he tells his brothers that he -

"...also saw that the justice of God did also divide the wicked from the righteous; and the brightness thereof was like unto the brightness of a flaming fire, which ascendeth up unto God forever and ever, and hath no end." (Book of Mormon | 1 Nephi 15:30)