In any Bible study we want to ask: What is being said?, Why is it being said?, and Why do I need to hear this? In a long and fairly complicated passage like this, the danger is that we spend most of the time on the first question, with a little bit on the second, and never get to the third question. Please avoid that.
Paul’s message to them at the start of their Christian lives had been about what Christ had done, not about anything they must do. God’s grace and blessing wasn’t something they had earned or merited, so nor should they imagine that the way to go on in the Christian life is tied to their efforts. Their own experience should have taught them that it is faith not works that is vital, as should their understanding of the Scriptures.
He reminds them of Abraham. The false teachers held that to be a true son of Abraham, people must submit to the Law (and so be circumcised), but Abraham wasn’t deemed righteous because of his Law-keeping (by that standard he was certainly not righteous); it was because of his faith that God counted him as if he were righteous. The true sons of Abraham, those who share in his blessing, are those who have faith and rely on that faith (not their performance) for their standing with God. Without full obedience, the Law only condemns (brings a curse). The blessings of the Gospel, the promise of the Spirit, come to us as we trust in Christ, who has borne the curse for us.
The Gospel is based on a promise, a promise God made to Abraham – actually to Abraham “and his seed”, which could mean his offspring in general (as in v.29), but the promise looked ahead to a particular offspring, namely Christ. The promise (v.8) was for Christ. The Law, given to Moses, came much later; but the Law didn’t supersede the promise (for a covenant promise cannot just be set aside), nor did it tweak the promise, introducing a new requirement of law keeping, because that would totally undermine the nature of the promise.
Whilst vv.19b-20 are fairly cryptic (I wouldn’t spend much if any time on them), vv.19-25 explain why the Law was given. It exposes what our hearts are like – that we are sinners in need of a saviour and unable to save ourselves. The law was like a gaoler, holding us in custody: it wasn’t meant to point the way to freedom but to show us our slavery to sin and the condemnation we deserved. And it was our “guardian” (v.24) – the word refers to a particular kind of household slave who would be responsible for a child until they grew to adulthood. It implies a temporary and preparatory role: to lead us to Christ, that we might put our trust in him. To go back to the Law is to give up freedom for slavery, and maturity for infancy. Why turn from Christ to the Law when the Law’s purpose is to tell people to go to Christ, in whom the promised blessing is to be found?
We are not likely to be tempted to embrace the Law in the way the original readers were (circumcision anyone?), but we are tempted to begin looking to our performance instead of looking to Christ and his work on the cross, or to think that the way we go on in the Christian life and know God’s blessing now is different from how we began the Christian life. And our wrong thinking is seen not only in how we think about our own relationship with God, but in how we view other Christians – judging them on the basis of their works (relative to our own), looking at them with an attitude of either spiritual superiority (looking down on them) or spiritual inferiority (feeling we’re hopeless in comparison).