Galatians 4: 30-31. “Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. So then, brethren, we are not children of the bondwoman, but of the free.”
The use of the word brethren in verse 31 makes it clear that Paul was writing to other believers; to those who had trusted in the amazing grace of God for salvation, and were not relying on the works of the flesh.
You see, it can’t be part grace, part works. Grace is an all-or-nothing thing. Grace would not be so amazing if it were conditional.