"So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do." Luke 17:10
"A creature as such owes its very existence, all that it is and has, to God; it cannot make any claims before God, and it cannot boast of anything; it has no rights and can make no demands of any kind. There is no such thing as merit in the existence of a creature before God, nor can there be since the relation between the Creator and a creature radically and once-and-for-all eliminates any notion of merit. This is true after the fall but no less before the fall. Then too, human beings were creatures, without entitlements, without rights, without merit. When we have done everything we have been instructed to do, we are still unworthy servants (douloi achreioi, Luke 17:10). Now, however, the religion of Holy Scripture is such that in it human beings can nevertheless, as it were, assert certain rights before God. For they have the freedom to come to him with prayer and thanksgiving, to address him as "Father," to take refuge in him in all circumstances of distress and death, to desire all good things from him, even to expect salvation and eternal life from him. All this is possible solely because God in his condescending goodness gives rights to his creatures. Every creaturely right is a given benefit, a gift of grace, undeserved and nonobligatory. All reward from the side of God originates in grace; no merit, either of condignity or of congruity, is possible. True religion, accordingly, cannot be anything other than a covenant: it has its origin in the condescending goodness and grace of God. It has that character before as well as after the fall."
Herman Bavinck, Reformed Dogmatics Vol. II, p.570