I think what Paul has in mind here is, because we’re proud people, we don’t really like to receive charity. The newspapers tell us there are people out there who are perfectly capable of working but choose to just live off benefits instead. All they do is take, take, take. And we would hate the idea that anyone could think of us in that category. We’ve worked to earn our living. We’ve contributed to society. We’re not freeloaders.
And we carry that mind-set into our religion.
We know we can’t obey the law to God’s standard of righteousness. We’ve all fallen short of the glory of God, so we know we need Christ to make up for our shortfall, but we don’t like the idea that all we do is take, take, take from God, so we want to do something to at least reduce our debt of unrighteousness a little bit.
The trouble is, in order to do that, we have to act as if we can produce some righteousness of our own, through obedience to the law.
And there are several problems with that.
Firstly, if we could have made ourselves righteous through the law, there would’ve been no point in Jesus dying for us (“if righteousness were through the law, then Christ died for no purpose”). Why would you send your son to die, if instead you could’ve just given people advice on how to obey the law better so they could make themselves righteous?
Secondly, if righteousness was achievable through the law, then God’s grace is worthless (“I do not nullify the grace of God”). For us to be able to think we’re making ourselves righteous by obeying the law, we have to reduce the law to something we can do. And when we reduce our view of the law, we reduce our view of our sin – how far we fall short of obedience to the law. And when we reduce our view of our sin, we don’t see the value of God’s grace. Romans 5v20: “the law came in to increase the trespass, but where sin increased, grace abounded all the more…”
When we understand the law properly, it will wipe out all hope of making ourselves righteous by obeying it, and that will lead us to realise more and more the value of God’s grace. It will magnify God’s grace, rather than nullifying it.