The book of Helaman portrays the people of Nephi dwindling in the strength of the Atonement. Due to their hearts turned away from God, they are limited to their own strength.
Helaman 4::12 "And it was because of the pride of their hearts, because of their exceeding riches, yea, it was because of their oppression to the poor, withholding their food from the hungry, withholding their clothing from the naked, and smiting their humble brethren upon the cheek, making a mock of that which was sacred, denying the spirit of prophecy and of revelation, murdering, plundering, lying, stealing, committing adultery, rising up in great contentions, and deserting away into the land of Nephi, among the Lamanites -- And because of this their great wickedness, and their boastings in their own strength, they were left in their own strength; therefore they did not prosper, but were afflicted and smitten, and driven, before the Lamanites, until they had lost possession of almost all their lands."
Without the grace of God, these people are left only with mortal strength which consists of might, cunning, deceit, and intellect.
Helaman 4:26 "For behold, they saw that the strength of the Lamanites was as great as their strength, even man for man. And thus had they fallen into this great transgression; yea, thus had they become weak, because of their transgression, in the space of not many years."
This was the spiritual condition that set the stage for the missionary work that Nephi and Lehi would do. As other leaders before them, these men left their administrative roles and went to teach individuals. And Lamanites, to boot.
Helaman 5:18 "And it came to pass that Nephi and Lehi did preach unto the Lamanites with such great power and authority, for they had power and authority given unto them that they might speak, and they also had what they should speak given unto them --"
Another great opportunity arises for the Lamanites to repent and come unto Christ and some of them have the sacred experience of seeing strength of the atonement come to the aid of Nephi and Lehi.
Helaman 5;37 "And it came to pass that this man did cry unto the multitude, that they might turn and look. And behold, there was power given unto them that they did turn and look; and they did behold the faces of Nephi and Lehi."
Take note that these Lamanites received power to see the physical miracle that comes over these missionaries. They see the fire of the spirit overcome the physical fire which threatened their lives.
Helaman 5:45 "And behold, the Holy Spirit of God did come down from heaven, and did enter into their hearts, and they were filled as if with fire, and they could speak forth marvelous words. And it came to pass that there came a voice unto them, yea, a pleasant voice, as if it were a whisper, saying: Peace, peace be unto you, because of your faith in my Well Beloved, who was from the foundation of the world. And now, when they heard this they cast up their eyes as if to behold from whence the voice came; and behold, they saw the heavens open; and angels came down out of heaven and ministered unto them."
Unfortunately, the Nephites did not see these miracles due to their pride and wickedness. Helaman 7:23 "For behold, thus saith the Lord: I will not show unto the wicked of my strength, to one more than the other, save it be unto those who repent of their sins, and hearken unto my words. Now therefore, I would that ye should behold, my brethren, that it shall be better for the Lamanites than for you except ye shall repent."
The Nephites are alone again in their own strength. They have only their plotting, coercion, and intrigue as tools to overcome their enemies. It takes faith in Jesus Christ to see the strength of his grace. Without faith the idea to lean upon God seems foolhardy when the need for power is essential. I long to be alive to that strength and to plead for it in my daily life.