The first question these verses raise in my mind is what does it mean that Abraham “believed God”? And the answer is not just that he believed certain facts about God. There was some communication going on between two people – God told Abraham something and Abraham trusted that it was true – He interacted with God and got to know aspects of His character, like how absolutely reliable His word is – that’s how God gets glorified; when people experience his glorious attributes.
When Paul says Abraham “believed God, and it was counted to him as righteousness”, he’s quoting Genesis 15:6. Abraham believed God when God said what? “Look towards heaven, and number the stars, if you are able to number them… So shall your offspring be.”
What was Abraham’s belief resting on? He simply believed that if God said it’s gonna happen, then it’s gonna happen – the promise wasn’t contingent on Abraham doing anything for God in return, and that chapter goes on to describe the covenant that Paul (as in Paul Oliver who preached here last Sunday) talked about, where the animals were cut in half and God passed between the pieces to illustrate the seriousness of the promise, effectively saying “let the same thing happen to me if I don’t keep this promise”.
I think the next question is what does it mean that Abraham’s belief was “counted to him as righteousness”? It means that faith in itself isn’t righteousness, but God counts people as righteous who aren’t righteous in themselves but who believe him – Abraham didn’t know about Jesus or the cross, but he put his trust in God’s promise for the future – the promise didn’t require Abraham to meet any conditions, so whatever God was going to do, it would be all God, and God would be the one earning all the glory – when God does all the work for us, it displays the immeasurable riches of his glorious grace – which is what God wants, hence his plan to make us righteous involves him doing all the work, paying all our debts himself, so that His grace is magnified, as opposed to a system where we contribute something to our righteousness, which would nullify His grace – see Ch2v21.)
Paul assumes the Galatians wanted to be sons of Abraham. It seems reasonable to assume that the people telling them they needed to be circumcised would’ve used Abraham to try to build their case for circumcision. One commentator pointed out that Christianity, Judaism and Islam all identify Abraham as a key figure in their history, so there are a lot of people in the world hoping to be blessed along with Abraham. And Paul asserts here that it’s those of faith – those who take God at his word like Abraham did – who are the sons of Abraham.