I searched my Bible app for the phrase “and to your offspring”, and it looks like Paul is referring to Genesis 13:14-15.
To be honest, I’m confused by the way he quotes Genesis here, because if you read on to Genesis 13:16, it’s clearly talking about multiple offspring. The word is literally translated “seed”, which doesn’t tell you whether it’s one or many. I’m using the ESV, which translates it “offspring”, which is at least as ambiguous as “seed”. But the translators of the New King James apparently ignored what Paul says in Galatians about the promise to Abraham, because they always translate it as Abraham’s “descendents” (plural).
If I understand Calvin correctly, he says the point Paul is making is that the promise was not to Abraham and all his biological descendents (the Jews would not have argued that either Ishmael’s or Esau’s descendents were blessed along with Abraham), but to Abraham and one specific descendent, Christ.
So our salvation doesn’t depend on being a biological or ceremonial descendent of Abraham, either by being born Jewish or performing religious rituals like circumcision. The important question for us is: are you in Christ?
We all need to have our own personal faith in Christ. None of us are saved because we were born into the right family or attend the right church; we each need to repent and believe the gospel for ourselves.
However, if we look at ourselves to try to work out whether we’re saved or not, there can be problems. The false teachers in Galatia taught that, to be a genuine Christian, you needed to be circumcised like them. If each of us is saved separately, our performance-wired brains are bound to start comparing our own performance with other people’s, to try to calculate how secure we are.
“He spends more time reading his Bible than I do.” “She does more evangelism than I do.” “I need to work harder to be like them to make sure God’s pleased with me and will accept me.” Or conversely, perhaps we think “I must be secure because I read my Bible and evangelise at least as much as them.” No. The promise was made to Christ. If we’re in Christ, we’re secure, and no more or less secure than anyone else who’s in Christ.