v3 – “I testify again” – Paul has already said this in chapter 3, but we’re liable to forget, and this is important: if you try to earn favour by your obedience on one point of the law, you’ll be held accountable for all of it.
Apparently people thought they needed to do this one thing (circumcision) and Christ would do the rest for them. They reduced the law to something they could do. That’s what legalists have to do to avoid despair.
Legalism and antinomianism appear to be opposite; one tries really hard to obey the law, while the other doesn’t worry about disobeying. But actually, they both have a low view of the law. The antinomian doesn’t think it matters too much if they break it, while the legalist has a low enough view of the law that they think they’re able to keep it.
If we have a right view of the law, we’ll realise how much trouble we’d be in if we were held accountable for the whole of it. We need Christ. Everything else has to go.
v4 – “You are severed from Christ, you who would be justified by the law” – You can’t sever something that wasn’t joined in the first place, so I think what Paul is talking about here is genuine Christians who are getting off track; there is hope for them to be restored, but they need to see where they’re going wrong.
In John 15 Jesus describes himself as the vine. If we’re connected to him, we’ll bear fruit, but apart from him we can do nothing. If faith doesn’t produce works, it’s evidently dead. But if we start relying on works, we cut ourselves off from the one who can make our faith alive.
“you have fallen away from grace” – It seems a bit harsh to say this about people who profess Christ and are just trying to add a bit of their own religion to make extra sure they’re saved, but justification in Christ and justification by works are mutually exclusive. You can not mix the two.
The fact that these people need to be told they’ve fallen away from grace indicates they don’t realise there’s anything wrong. It’s possible to fall away and not even realise it. You had a religious conversion experience once, and you keep busy with lots of Christian activities (Bible reading, going to church, leading Bible studies…) and don’t notice that you’ve departed from grace.
In fact, we might even be tempted to think “I’ve been baptised, I’ve been a member of the right kind of church for decades, I read my Bible and pray every day, I try to evangelise, so I clearly haven’t fallen away from grace; this doesn’t apply to me”, but Paul’s reasoning doesn’t work that way.
In fact, it’s the opposite. It’s precisely the religious people who’ve ticked all the right boxes, and who look to those things for their confidence of salvation, that Paul is talking about. They are the ones who have fallen away from grace.