In fact, those who insist that Scripture's use of "reward," of "crown," of "inheritance," and similar imagery proves that merit is involved in human obedience and perseverance actually join ranks with those folks Martin Luther called the Papists. This is precisely the argument that Luther is countering in his response to Erasmus in the portion I excerpt below.
Yesterday, during my writing on the issue, I had occasion to be reminded of the following excerpt from Luther's response to Erasmus in The Bondage of the Will (183-184) which I accessed. When I first read the book more than thirty years ago I underlined and marked this portion and others as highly significant.
What then is the meaning of all those Scriptrues which promise the kingdom and threaten hell? Why is the word 'reward' repeated so often in the Scriptures? 'There is a reward for thy work' (2 Chron. 15.7). 'I am thy exceeding great reward' (Gen. 15.1). Again: 'Who rendereth to every man according to his work' (cf. Job 34.11). And Paul says in Rom. 2: 'To those who by patient continuance in well-doing seek eternal life' (v. 7); and there are many similar statements. The answer is that what is established by all these passages is simply a consequence of reward, not in any way a worthiness of merit; inasmuch as those who do good do not do so in a servile, mercenary spirit, whith a view to gaining eternal life--although they seek eternal life in the sense that they are in the way by which they will find and attain eternal life; so that their 'seeking' is an earnest striving and diligent endeavour after that which regularly follows upon a good life. The reason why the future consequences of a good and bad life are declared in the Scriptures is that men might be instructed, disturbed, awakened and terrified. As 'by the law is the knowledge of sin,' and instruction concerning our impotence--by which, however, it is not implied that we ourselves can do anything; so by these promises and threats comes a warning of what follows upon the sin and impotence which the law has pointed out--but they do not ascribe any worthiness to our merit. Wherefore, as the words of the law serve their turn by instruction and illumination, to teach us both what we ought to do and what we cannot do, so the words of reward, signifying what is to be, serve their turn by exhorting and threatening, and animate, comfort and uphold the godly to press on, persevere and triumph indoing good and enduring evil, lest they should be wearied, or their spirit broken. So Paul exhorts his Corinthian converts, saying: 'Quit you like men, knowing that your labour is not in vain in the Lord' (cf. 1 Cor. 16.13, 15.58). So also God upholds Abraham, saying: 'I am thy exceeding great reward' (Gen. 15.1). It is just as if you were to comfort someone by intimating to him that his works certainly please God. This is a kind of consolation which Scripture often employs. And it is no small consolation to know that one pleases God, so that nothing untoward can follow, impossible though that may seem to be.
Let it be known, thus, that when I make the claim that Tom Schreiner and I present the classic doctrine concerning warnings and admonitions in The Race Set Before Us, I have solid basis for making the claim. Not only does John Calvin agree with us, if you can bear the anachronism, Martin Luther does also. For additional extensive quotes that show that what we affirm is Classic Reformed teaching, see Run to Win the Prize by Tom Schreiner.