28 For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh;
Verses 28 and 29 are the conclusion and climax to the argument beginning at verse 17. The argument was that a true Jew is not one who honors the external but one who operates by genuine, internal reality. Note the contrasts in verses 28 and 29:
from men—from God
The “for” here demonstrates who is the true believer. Even a Gentile who he is not circumcised can believe with his heart. The word “for” shows why outward religious experience does not make one right with God.
This verse shows the negative side of Paul’s argument—what a true Jew is not.
he is not a Jew who is one outwardly [in the public domain], nor is circumcision that which is outward [clear, plain] in the flesh [physical];
A true Jew is not one who has been circumcised physically by cutting the foreskin of the penis, but one who truly applies Scripture to experience. Just because a person is circumcised does not mean that he is a true Jew as far as eternal life is concerned.
God rejects formalism without reality.
Many people trust in liturgy without change of heart. This is formalism without reality, a form of self-deception. They are clean on the outside but corrupt on the inside. Ritualism is a form of spiritual laziness; it takes a serious application of truth to experience to mature as a believer.
God loves those with a vital faith. It take rigorous understanding of the principles of Scripture and applying them to experience to possess a dynamic faith. Form without faith requires little of us.