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953 B
outwardly
This refers to Jewish rituals which people can see.
outward
This is the physical evidence on his genitals that a man has received circumcision.
he is a Jew who is one inwardly, and circumcision is that of the heart
This is a doublet in which "he is a Jew who is one inwardly" explains the metaphor "circumcision is that of the heart."
inwardly
This refers to the values and motivations of the person whom God has transformed.
in the spirit
This probably refers to the internal, spiritual part of a person, in contrast to the outward "letter" of the Law. However, it is also possible that it refers to the Holy Spirit (see UDB).
not in the letter
A “letter” is the smallest part of written language. Here it refers to written Scripture. Alternative translation: “through the work of the Holy Spirit, not because you know the scriptures.” (See: rc://en/ta/man/translate/figs-synecdoche)