I freely and cheerfully grant that the Jews of the first century did not hold the peculiar merit theology that prevailed during the medieval era. But that particular merit theology is only one of countless ways that fallen men have devised to boast in themselves instead of in the Lord. And the problem of this boasting is not addressed if the boaster acknowledges that the Lord is responsible for putting the prerequisite events in motion. If God starts it with His grace, and man finishes the job, there is boasting in man. If God is never acknowledged, and man does it all, there is boasting. If God does it all, and man takes pride in knowing that, there is boasting.
The apostolic question is this: “Where is boasting?” When we see the smoke of boasting, we may then look for the fire, and we also may ask what kind of fire it might be. It might be the fire of tribalism, or racialism, or moralism, or orthodoxyism. If you prove to me that medieval merit mongers and first century Pharisees had different mechanisms for robbing God of His glory in salvation, I grant the point, and continue to maintain that it is beside the point.
Long before the days of mendicant friars, the New Testament writers are in a fearful battle with a religious establishment that did not boast in the Lord, regardless of what they said they did. If the New Testament is a faithful representative of these adversaries (and it is), then we have to say that they were arrogating to themselves something that only God should receive. We may call this self-arrogation any number of things, so long as we are not making a precise equivalent (with regard to methodology) to what their soteriological cousins would be doing centuries later. But on a basic level, if you are not boasting in the Lord (really), then you are somehow “taking credit,” or “earning,” or “striving,” or “accumulating merit.”
In all ages, in all times, in all cultures, with all religious terminology, there have only been two options, which are boasting in the Lord and boasting in man. So let boasting in man serve as the genus, and be as particular as you like about the variants between the species. It does no good to deny that the Pharisees believed in merit. Of course they did not, but they did believe in themselves. And that was their problem. And it turns out it was the same problem that showed up in the medieval era.
We don’t have this difficulty on other subjects, and I believe that this indicates the presence of a serious spiritual confusion. Nobody thinks to say that Cain was not really a murderer in the modern sense because he did not use a gun.