A person can have nothing more beneficial than sincerity towards his Lord in all his affairs, along with sincerity of resolve, so he should be sincere towards Him in his resolve and in his actions. Allaah says:
‘And when the matter (preparation for Jihaad) is resolved on, then if they had been true to Allaah, it would have been better for them‘ [Muhammad 47:21].
So his happiness lies in sincerity of resolve and sincerity of action. Sincerity of resolve means certain desire to do an action and not hesitating to do it. If his resolve is sincere, all that is left is sincerity of action, which is doing one’s best and striving one’s utmost when doing it and not letting anything, outward or inward, hold him back from it. Resolve of purpose prevents him from becoming weak-willed and sincerity of action prevents him from becoming lazy or lethargic. If a person is sincere towards Allah in all his affairs, Allah will give him more than He gives to others, and this sincerity results from true love of Allah and true trust in Him. So the most sincere of people is the one who is loves Allah most and puts most trust in Him.