The concept of esoteric—“inner”—teachings or practices has an important role in the history of religions. The fundamental idea is that there are certain exoteric beliefs and practices that are public taught and widely known, while there are other esoteric ideas and practices that are restricted to the prepared, enlightened, initiated, educated, or spiritually advanced.
The English terms esoteric and exoteric derive from Greek roots, but are cognate with the more straightforward Latinate interior and exterior, or more generally in English, inner and outer. The Greek terms esōteros and esōterikos, however, were rarely used for inner and out religious teachings in classical antiquity.
From the biblical perspective, however, the Greek term esōteros is a rare term with a very special technical meaning. This can be seen in its major occurrence of the word in the New Testament in Heb 6:19. Here Paul is discussing Christ, as the Great High Priest, entering the Holy of Holies of the celestial Temple.
In the KJV it reads: “Which hope we have as an anchor of the soul, both sure and steadfast, and which entereth into that within the veil”
The NRSV reads: “We have this hope, a sure and steadfast anchor of the soul, a hope that enters the inner shrine behind the curtain”
The Greek for the bold face phrase here is esōteron tou katapetasmatos. Katapetasma is the technical term in both the LXX and the New Testament for the veil or curtain of the Temple. Esōteron means literally, as the KJV has it, “that [place/thing] which is within,” and, as noted earlier, is the Greek root from which we draw our word esoteric. (The meanings of these words are taken from the Greek-English Lexicon of the New Testament, 3rd ed., and the Greek-English Lexicon of the Septuagint, 2nd ed.)
However, as the NRSV rightly translates esōteron more technically as the “inner shrine behind” (or better “within) the curtain/veil (katapetasma).
The NRSV translates esōteron as the “inner shrine behind/within” because Paul is, in fact, quoting a technical use of that phrase from the description of the Day of Atonement ritual in Lev 16, where the Holy of Holies within the temple veil is consistently described in the Septuagint as esōteron tou katapetasmatos (Lev 16:2, 12, 15). In biblical Greek, then, esōteron tou katapetasmatos is a technical phrase meaning “[the place of the Holy of Holies] within the veil/curtain.”
Esoteric, then, in its original biblical meaning, refers to the teachings and practices done within the veil of the Temple. This concept helps us understand that in the biblical world view there were public, exoteric rites and teachings performed in the outer court of the Temple in view of all the people, and there were private, esoteric rites and teachings performed within the Temple building and restricted to the priests or even the High Priest alone.