First Nephi 3:7
There are certain attributes God will always have, there are certain things he will always do, and there are things he will never do. Knowing these constancies allows us to have faith in Him. One of the things God will never do is force us to disobey him. To create a scenario where we can't obey would make him responsible for our disobedience. He would cease to be God. Commandments may seem impossible to us, but if the Lord has commanded it, it is always possible. It must be. If we are to be punished for disobedience, it is our fault. In the same way, the Lord will never force us to obey either.
Laman and Lemuel thought getting the brass plates was impossible. Nephi knew that because the Lord had commanded it, he would prepare a way.