Albert Barnes on Galatians 3:9

So then they which be of faith. They whose leading characteristic it is that they believe. This was the leading trait in the character of Abraham; and this is the leading thing required of those who embrace the gospel, and in the character of a true Christian.

Are blessed with faithful Abraham. In the same manner they are interested in the promises made to him, and they will be treated as he was. They are justified in the same manner, and admitted to the same privileges on earth and in heaven.

(f) "then they" De 27:26