Why was 3 John included in the Bible?

Historically, 3 John was written after the final revelation to John. Why was it still added to the Bible?

First, it is important to note that we do not know the exact order of the writings of John. We also do not know the exact dates. The Gospel of John has been proposed to have been written from as early a 65 AD to as late as 90 AD. Additionally, 1, 2 and 3 John have been proposed from 70 AD to about 110 AD. Revelation is typically placed between 90 and 95 AD. Both 2 and 3 John make reference to previous writings (likely referring to the Gospel of John). Traditionally, it is believed that John died between 95 AD and sometime in the early part of the second century (between 100 and 110 AD).

We do know historically that John was placed in exile on the Island of Patmos where he wrote the book of Revelation. We also know that he was released and went back to Ephesus after he served his sentence. He died in Ephesus a short time later.

Because we know that, in 3 John, he speaks of his desire to visit people, we know he was still travelling. We do not know if this was a desire to visit the community to which he is writing before he was put in exile, a desire to visit them upon his release, or simply a community he desired to visit after his release.

I think the heart of your question really has to be answered in light of Revelation 22:18-19:

¹⁸ I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll. ¹⁹ And if anyone takes words away from this scroll of prophecy, God will take away from that person any share in the tree of life and in the Holy City, which are described in this scroll.

If we believe that the warning to refrain from adding to the scroll is meant for the Book of Revelation alone, there is no problem including 3 John in the Canon even if it was written after Revelation. If we believe that the warning includes all of the Bible, then we must either believe that 3 John was written before Revelation or that the verse was not applied to all of the Canon until after the books were compiled into a single volume.

If we believe that the warning to refrain from adding to the scroll is meant for the Book of Revelation alone, there is no problem including 3 John in the Canon even if it was written after Revelation

The best way to understand the verse is to believe that it was in reference specifically to the Book of Revelation. Revelation was written on a scroll and sent to the churches. It includes the final Revelation of Christ because it discusses his return, judgment, and the institution of the eternal state. Therefore, there is no need to add anything else to the Gospel message. Third John is written to a friend in a community of faith and John warns him and the congregation not to give in to evil. Like the other letters of John, it has to do more with Christian living than the revelation of Christ or the Gospel.

We also know historically that the writings of the Apostles were considered equivalent to the writings of the Old Testament Prophets. This means that they were treated as Scripture.

In 2 Peter 3, we find that Peter views the writings of Paul to be Scripture:

¹⁵ Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. ¹⁶ He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.

Just as Peter views Paul’s writings to be Scripture, so the early church viewed John’s writings as Scripture. As a result, all three letters, the Gospel, and the Book of Revelation are included in the Bible today.

While I believe that John likely wrote 1, 2, and 3 John before he wrote Revelation, I also believe he wrote all of his writings near the same time towards the end of his life. The internal evidence from all three points to a unified author. There is no reason to doubt the authorship of any of these texts or to think that one should not be in the Bible. The messages of all are consistent and they are all meant to give hope to believers struggling to live out their faith in an environment where they face persecution from both the public and the government.