[3:171] Joyful in grace and bounty from God, and that God does not let the wage of believers go to waste.
The occasion of their being joyful and its cause is a favor and act of grace from God, i.e., how could they be joyful if not for His favor and grace to them? Their being joyful through the act of grace is only because they are His servants and He is their Protector (mawlā). If not for His grace and favor to them, they would not be in this state.