Helaman 15:10

And now because of their steadfastness when they do believe in that thing which they do believe; for because of their firmness when they are once enlightened, behold the Lord shall bless them and prolong their days, notwithstanding their iniquity;

They are steady when they believe something. They are firm once they understand the truth. Because of this, the Lord will bless them. He will let them live longer. This is true even when they do wrong.

Because of their commitment—when they believe something, they really believe it—because of how firm they are once they see the light, the Lord will bless them and give them long life, even despite their past mistakes.