What does it mean to have a sincere intention?

{ And they were not commanded except to worship Allah, [being] sincere to Him in religion, inclining to truth, and to establish prayer and to give zakah. And that is the correct religion.}

{ And not [giving] for anyone who has [done him] a favor to be rewarded But only seeking the countenance of his Lord, Most High. }

{ [Saying], "We feed you only for the countenance of Allah . We wish not from you reward or gratitude.}

{ Whoever desires the harvest of the Hereafter - We increase for him in his harvest. And whoever desires the harvest of this world - We give him thereof, but there is not for him in the Hereafter any share. }