Christians are people set free - Paul sets out his gospel as freedom from this present evil age in 1v3-5. A present evil age that is not so much this world as the slavery of religious zeal, idolatry or law observance. He's shown extensively that it is absurd and evil for a Christian to forsake this freedom to become enslaved again.
A few last reasons -
- 5v2, the free person enslaved will find Christ to be of no advantage to them.
- 5v3, when the slavery embraced is law it carries an in built obligation to keep all of the law...
- 5v4, the path of slavery severs the free person from the source of their freedom, namely Christ.
The compulsion to slavery might seem like a small detail, circumcision, a food law, some other small safety net in addition to Christ. But, the smallest addition spreads like yeast in a batch and has great consequences. Paul however never endorses even the most minor slavery. What's the proof? He's being persecuted and people don't tend to persecute teachers of slavery. It seems counter-intuitive that slavery would be popular but most of us like to make some contribution to our standing before. God. The offense is found in the Cross which allows us no contribution. It requires nothing additional - because it is utterly sufficient to set us free!
In Galatians we find Paul bewildered, astonished, perplexed and angered. Gospel-denial is the one thing above all others that enrages him. Such is his concern for the glorious gospel. We see this in action as we looks at Athens enslaved to idols and is provoked. Not provoked to destruction but provoked to reason with people about the Cross. But here in 5v12 he is infuriated with false teachers - people who pretend to teach the Cross but lay extra burdens on people.
How then should a Christian live? We saw Peter not wanting to sin and that was a good desire. Law shouldn't be used to control sin, but neither should freedom be used to indulge sin (whether in legalism or idolatry or otherwise). Christians are not compelled to sin. Sin is not something that has to be done by a believer - it's enslavement is ended. Sin is now essentially voluntary but that is a freedom to be avoided.
Instead our freedom is an opportunity to serve others in love. To do, ironically, the very thing that any Jew could tell you is the summary of the law - to love your neighbour as yourself. But this isn't brute force law-keeping by the backdoor. Faith works out through love (v6), and by the Holy Spirit (v5). The believer takes action to love and the Holy Spirit acts to produce love. Love, patience, kindness and more.
The law was a prison and slave master to Israel and yet here we see it in a more positive light. This is unexpected! The new free life of a Christian is wrapped up with God. The Holy Spirit lives in the believer - the key benefit of the gospel - as Christ is being formed within. Christ's righteousness is counted to the believer and Christ is the clothing the believer wears. That kind of life evidently looks like a law-kept life. The power and motivation are very different and the goal is higher. Not slavelike obedience but Spirit-filled, Spirit-empowered living.