The controversy Paul is dealing with in Galatians had to do with whether Gentiles must become Jews in order to be saved. The Judaizer’s believed you must do something to be saved such as keep the law and be circumcised. Today many teach you must be baptized or take communion, keep the law, do more good than bad etc. All of these things are considered works that will not save anyone. The promise in this verse is the same as it was to Abraham, you must come to Christ by faith plus nothing else. The only way to be saved is to believe on the Lord Jesus Christ and you will be saved. (Romans 10:9-10) Abraham believed and it was accounted to him for righteousness. (Genesis 15:6, Romans 4:3) this means that God promised Abraham that all who believe will become a spiritual descendant of Abraham, in other words a saved person and they become heirs according to the promise made to Abraham that all who believe in Christ will be saved. Have you come to Christ by faith, nothing else will save you?
Galatians 3:29 And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise.