Paul is saying to the Jews & Gentiles who are you to judge someone else given our own sins and brokenness before God. Our/my failure to life up to God’s Law.  Paul doesn’t put much value in those people (Christian/Jew) that understand, knows, and study God’s law, are circumcised as the essence of righteous relationship with God.  It is only meaningful if you obey God’s law.  It’s the act of doing God’s law/ways that is pleasing to God.  Verse 14 and 28 makes this very clear.

14 Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.”

28 A person is not a Jew who is one only outwardly, nor is circumcision merely outward and physical. No, a person is a Jew who is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a person’s praise is not from other people, but from God.

There is a powerful simplicity in following Jesus. The cost is giving myself/yourself completely to God’s will.