And many more such things did he say unto them, telling them that there could be no atonement made for the sins of men, but every man fared in this life according to the management of the creature; therefore every man prospered according to his genius, and that every man conquered according to his strength; and whatsoever a man did was no crime.

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.

COMMENTARY

But every man fared prospered according to his genius, conquered according to his strength
And because of their boastings in their own strength, they were left in their own strength
To the ancient Nephites a false prophet came, denying the existence of Christ, and teaching the people that reliance upon God was foolhardy. Korihor taught that each person had only him- or herself to depend on. If you were to succeed or fail was entirely up to you, and you alone.
As the people came to embrace this philosophy, it actually became a self-fulfilling prophecy. Having rejected God, they became Godless. Having insisted upon relying on their own strength only, they were left to their own strength only. Thus, in a sense they were absolutely right in their beliefs. By denying the existence of God, they ensured that in essence He would not exist (for them).
Of course we cannot change eternal truths by our opinions, we do not have that power. But we do have power over the individual experience of our lives. We have the ability to invite or reject divine intervention. We have the ability to put up or break down walls. We can make our portion of the world a place that is hopeful and blessed, or we can make it into a place that is cynical and isolated. Given that we have this power, we ought to take great care in which reality we are electing for ourselves.