Galatians 4:21-5:12 (AD 48)
The two parts of this passage are where Paul continued the theme of grace versus the law that was his focus since chapter two. You – he told the Christians – have a gift in the same divine way that Abraham and Sarah had a gift. Not by trying to force God’s hand, like Sarah giving her maidservant to Abraham to continue the family line. But a straight-up miracle from God as Isaac eventually was.
You want to force God’s hand, Paul scoffed from v.21. You think you can be good enough to make him accept you, like Sarah and Abraham thought God needed a push before he would fulfil his promise to them? That only ended badly; the descendants of Hagar’s son Ishmael persecuted the descendants of Isaac (ref e.g. Gen 25:18). And in the days of the early church the proponents of the Law (Judaisers) were persecuting the Christians.
WHY WOULD YOU DO THAT?! Why would you try and get to God by impressing him or forcing his hand? What about our entire history would make you think that’s a good idea? Why, furthermore, would you effectively enslave yourself? Our history involves being enslaved, and God rescuing us from slavery…and now you want to go back?!

Paul continued, You think you’ll pressure God by obeying the law of circumcision? Well if you’re going that route you can’t stop there – there are a whole lot more rules and regulations where that came from, so you’ll need to obey all of them, not just that one. If you add one rule to grace (thus destroying grace), you have to add all of them. You’ve disappeared from Jesus, you’ve lost him entirely, you’re now in the world of be-perfect-or-be-damned, Paul said, and you can’t win.
Paul described those introducing this enslaving, false, damnable teaching of salvation by works, as those who “cut in on you” (5:7), as someone who was “throwing [them] into confusion” who “will have to pay the penalty” (v.10). He called them “agitators” who he wished would take their argument to its logical conclusion: cut off their own penis – i.e. completely un-man themselves (v.12). That’s how self-defeating and idiotic their doctrine was. That’s how crazy it was for the Galatians to fall into lock-step behind them.
Their teaching was as ridiculous then as it is common now, and it’s still couched in, ‘You need to repent and trust God, and…’. The ink drop is still being dropped into the water, the only-one-hole is still being pricked into the balloon, and people still don’t understand why that’s a problem.
So why enslave yourself to false hope? Why reach for the stars of moral perfection when you cannot get the moon, only the grisly end of abject failure? Why not allow the grace of God through the perfection of our Saviour Jesus, to be your everything?
Or, to jump back into Paul’s Old Testament metaphor: Don’t be like a ninety year old man who slept with a young woman to try and force God to do things his way. It didn’t look good, nobody appreciated it, and it ended badly for everyone.