These two phrases have similar meanings. The first phrase, "he is a Jew who is one inwardly," explains the second phrase, "circumcision is that of the heart." (See: [[rc://en/ta/man/jit/figs-parallelism]])
Here "letter" is a synecdoche that refers to written Scripture. Alternate translation: "through the work of the Holy Spirit, not because you know the Scriptures" (See: [[rc://en/ta/man/jit/figs-synecdoche]])