Matthew Henry: If we tried to contribute our own works to our justification through Christ but failed, that would mean Christ was a servant of sin. That could not possibly be the case. Hence we cannot contribute our own works to our justification.
Calvin: If Christ did away with justification by the law, does that mean Jews who would otherwise have been justified by the law are now classed as sinners, and does that make Christ the servant of sin? No, Christ only revealed sin that was already there.
Keller: If someone professes to be justified in Christ but continues in their life of sin, does that make Christ a servant of sin? No. If someone continues in their old sinful lifestyle, they prove that they weren’t in Christ in the first place.
The interpretation that makes most sense to me is: It seems that Paul is responding to some people who think we should teach that Christians need to contribute some works of the law to their justification, because they’re worried that preaching this message of justification by faith makes Christ a servant of sin.
They seem to be concerned about people who show an interest in Christianity – they “seek to be justified in Christ” – but who aren’t living the way you’d expect Christians to live – they’re “found to be sinners”.
And they seem to be concerned that this phenomenon where people are seeking to be justified in Christ but are then carrying on living in sin is connected with, even promoted by, the message of justification by faith.
If we’re justified by faith, not works of the law, why would people bother doing the works of the law at all? Why wouldn’t they just carry on enjoying their sin?
And if preaching a message of justification by faith in the name of Christ encourages people to sin, doesn’t that make Christ a servant of sin?
Paul says that’s not what happens.
I think when he says “If I rebuild what I tore down, I prove myself to be a transgressor…” he means that if we rebuild our old lives, we prove that we were never truly in Christ in the first place. He goes on to say (v19) “For through the law I died to the law, so that I might live to God. I have been crucified with Christ. It is no longer I who live, but Christ who lives in me…” If I rebuild my old life of sin, it shows that I didn’t really die to it. I wasn’t really crucified with Christ.
It’s not the message of justification by faith that’s the problem.
So if someone claims to be a Christian – perhaps they even tore down their old lives at some point (they turned over a new leaf) – but now they’re rebuilding their old lives of sin, we might be tempted to try and get them to change their behaviour by modifying the gospel, adding some works of the law that they must do or they won’t be justified, but that’s not going to help them. They don’t need works of the law; they need to die to the law, and be crucified with Christ so that they might live to God.
For someone to “endeavour to be justified in Christ” doesn’t really make sense. Either you put your faith in his righteousness to justify you, or you don’t.
To some people it seems that preaching the gospel of justification by faith encourages Christians to sin. Matthew Henry said this was not a new accusation in his day, but it is an unjust one, because any true Christian will hate the thought of making Christ a servant of their sin.