First Nephi 7:12
The Lord is able. In the verse previous to this, Nephi gives the example of getting the brass plates from Laban. Laman and Lemuel thought that Laban and his fifty were too powerful. To them it was an impossible task. Their failure to understand what Nephi understood ensured the impossibility of them ever succeeding in obtaining the plates: In order for the Lord to do all things for his children, they must first exercise faith in him.
Truly it was impossible...for Laman and Lemuel. It was impossible not because the Lord is not able, not because Laban was more powerful, but because Laman and Lemuel had no faith. For Nephi, it was possible. Because of Nephi's faith in the Lord, the Lord was able to help him.
The Lord can't do things for his children if they have no faith in Him. Not because the Lord doesn't have the ability, but because we don't have the currency: faith. Before we ask: "Is the Lord able to do this thing?" We should ask: "Do we have faith in him?" If so, then yes, if not, then no.