To begin with, “Jo” is not the abbreviation for John. It’s “Jn”. (I first looked up Joel.) Context is most important for understanding anything—especially Scripture. The passage at hand is situated in the context of a warning against being taken up with the teaching that Jesus is not the Christ. “The man who denies that Jesus is the Christ—he is a liar, he is Antichrist;” (1Jn 3: 22) He then goes on to say that he is writing about people who are out to lead them astray. “Keep alive in yourselves what you were taught in the beginning: as long as what you were taught in the beginning is alive in you, you will live in the Son and in the Father; and what is promised to you by his own promise is eternal life.” (1 Jn 2:24)
What was taught to them in the beginning was Sacred Tradition—not Sacred Scripture. The New Testament didn’t exist yet. They are being urged to keep faithful to what they were taught by word of mouth through the power of the Holy Spirit. It is through the power of the Holy Spirit that they have the gift of faith and they are being cautioned to not lose that gift. This is the Revised Standard Bible translation: “But you have been anointed by the Holy One, and you all know.” That is, they know the truth of what they have been taught—by the power of the Holy Spirit. This in no way was an endorsement of the infallibility of individuals to interpret Christian Scripture when it is eventually compiled in the future.
Fr. Vincent Serpa, O.P.