I'm reading Calvin [below] as saying that the Father was the primary cause of the Son's obedience. The Son had a mission to fulfill, and He did it. He had stuff to do. But He did not have to earn God's favor, like he didn't have it before. Same with Adam, pre-Fall. It does not follow that if God does everything for Adam, Jesus or for us in grace, that what is done is worthless. Those works mentioned in Rev 19:8 were only possible by grace, but certainly not seen there as worthless. Our labor is not in vain, and yet is all of God's grace. We work out our salvation b/c God works in us to will and do it.
If merit means fulfilling the terms of a gracious covenant, I'm fine with it. In that sense, merit and grace can co-exist. We merit salvation by exercising faith in Christ. FV thinks saying "living-faith-which-includes-works" is much better than saying "merit," in this context, since merit implies lack of gracious covenant terms. Creating someone/thing and establishing a covenant with them is inherently gracious.
But usually merit means work(s), and DOES rule out grace. Rom 11:6. The word is specifically used to deny that the cov of works was gracious. This is the anti-FV version of the cov of works: no grace involved, b/c it is works and Adam merits or demerits the tree of life, sans grace. It is on-his-own obedience. Except that it wasn't.
Merit implies on-his-own. Faithfulness emphasizes the relationship the parties are in from the beginning... much better.
What is lost saying "faithful in covenant" instead of "merited terms of covenant"? Only an unbiblical, on-your-own, earn-your-way-into-God's-favor posture. And we gain the relational aspect b/t God and Adam pre-Fall.
This is not a false dichotomy, between contract and covenant relationship, but a spectrum which shades how we view our relationship with God. Very important.
Why so much emphasis on the (contractual) commands and prohibitions and so little on the (covenantal) walking together in the cool of the day and hiding from God in shame? Why so much emphasis on the Sinai laws and so little on the covenantal presence in tabernacle. Both are present, the contractual flowing out of the relational. We obey out of gratitude, not out of guilt or law-contract.
If your son merits his allowance, he need not be grateful to you. He earned it and deserves it. You owe it to him.
But if your son is faithful to you, he remains grateful.
The allowance contract is there as one aspect of the father/son relationship, which is more fundamental.
I don't like the contract emphasis, but agree that element is necessary. God is ultimately to us a Father, not a lawyer or business partner. Forensic justification is real and necessary, but it is there so we can leave the courtroom and go home with our Father and eat at His table.
Emphasis will depend on audience. General world or soft evangelicals need to hear the backbone of grace: law, sin, atonement, etc. Conservative, strict, Reformed types usually need to hear covenant as relationship, and stay away from the idea that the essence of the Gospel is the forensic and legal, while the relationship is icing on the cake. The Gospel is both escape from a justly wrathful God, and a Forgiving Father welcoming home a prodigal son.