I think its important when dealing with Paul's writings or anyone's writings but especially Paul's since it came with such a stern warning 2 Peter 3:15-16 to really prayerfully study what he is saying, compare it with other scripture and make sure it doesn't contradict the teachings of Jesus such as Mat 5:17-30 Mat 15:3-14 Mark 7:7-13 Mat 19:17-19 John 14:15 and even Paul's own teachings where he says the commandments are holy, just and good (when should we not want to be like this) or breaking the commandments shows us our sins Rom 7:7 and breaking them dishonors God Rom 2:22-23 how we love our neighbor and God Rom 13:9 1 John 5:2-3 and keeping God's commandments is what matters 1 Cor 7:19
Paul is contrasting the two covenants- the old covenant verses the new and using an allegory of Sarah and Hagar.
First we need to understand what the new covenant is established on.
PROMISES- not new or better laws- we can't make something God wrote that is perfect for converting our souls more perfect Psa 19:7
Hebrews 8:6 But now He has obtained a more excellent ministry, inasmuch as He is also Mediator of a better covenant, which was
established on better promises.
This is what the promise was of the first covenant
Exo 19: 8 Then all the people answered together and said, “All that the Lord has spoken
we will do.” So Moses brought back the words of the people to the Lord.
So the people are doing in the OC when they promised to keep God's covenant.
This is what the New Covenant is established on the better promise:
Heb 8:10 For this
is the covenant that I will make with the house of Israel after those days, says the Lord:
I will put My laws in their mind and write them on their hearts; and I will be their God, and they shall be My people.
So in the NC it is no longer established on our strength of obeying God, its based on His strength - He is the one doing- we just need to cooperate with Him.
John 14:15
“If you love Me, keep My commandments. 16
And I will pray the Father,
and He will give you another [
e]Helper, that He may abide with you forever— 17 the Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you and will be in you. 18 I will not leave you orphans; I will come to you.
So its a much better promise having God doing, than relying on our own strength.
How does these better promises relate to Sarah and Hagar. It relates perfectly.
Abraham was promised to make a great nation through his seed, but the problem was Sarah was in advanced age Genesis 18:11 and instead of depending on God's strength she took matters in her own hands by taking their slave Hagar, to bear a child for Abraham instead of trusting God that He would deliver on His promise in His own time.