Luke 17:7-10

That works do not justify. 'Our Lord having exhorted His disciples to good works, now proceeds to rebuke the vainglory which so often accompanies them, showing that as a master is under no obligation to a slave who performs his appointed tasks, so neither is God to us. But since God is gracious, He treats those who are slaves, as if they were free hired labourers, and recompenses their labours with a reward, and receives their service which is strictly due, as if it were meritorious, and gives a requital out of all proportion to the toil. Thus the goodness of God is stronger than His justice' (Euthymius).