Albert Barnes on Galatians 3:12

And the law is not of faith. The law is not a matter of faith; it does not relate to faith; it does not require faith; it deals in other matters, and it pertains to another system than to faith.

But, The man, etc. This is the language of the law, and this is what the law teaches. It does not make provision for faith, but it requires unwavering and perpetual obedience, if man would obtain life by it. See this passage explained Rom 10:5

(b) "law is not of faith" Rom 10:5,6 (c) "The man that" Lev 18:5, Eze 20:11 (*) "live in them" "by"