Faith.

To perform any work requires that we have faith in the ultimate result of that work. No farmer would plant unless he expected to reap; no builder would build, unless he expected one to inhabit; no speculator would invest, unless he expected to increase his means; no journey would be attempted unless there existed hope of reaching the destination.

So, likewise, no commandment of God would be obeyed unless there existed faith that certain blessings would follow obedience.