I always wonder what Paul was thinking as He wrote an epistle. Was he sitting there thinking “man this is good stuff” or was he completely unaware that what he was writing would be read by billions? I’m sure that he wrote other stuff outside of what is now Scripture. I wonder if he felt any different when writing that, as opposed to his Holy Spirit-inspired letters. Perhaps a warm, tingling sensation to let him know when he was inspired. If we look at Paul’s writings we know for sure that the thirteen Pauline Epistles are not all he wrote. There are a couple of writings that were written by authors of the New Testament, but just didn’t make the cut for the Bible.
The most well known of the “lost books” of the New Testament were written by Paul to the Corinthians. In 1 Corinthians 5:9, we find out that this is not Paul’s first correspondence with the Christians of Corinth. Paul founded the Church there in Acts 18:1-11 and then stayed in Corinth for a year and a half. Sometime after that and before writing 1 Corinthians, Paul wrote the actual first letter to the Corinthians. Looking further we see that not only was there one lost letter, but 2 Corinthians 2:3-4 and 2 Corinthians 7:8-12 indicates a second lost letter. Some think that these verses only point to 1 Corinthians, but the timeline seems to indicate that it is a fourth letter. 
In 2 Corinthians, Paul references a visit that was painful, yet it was apparently short enough not to be mentioned in Acts. This painful visit is not mentioned in 1 Corinthians, so it was likely made sometime after. So in reality our 1 and 2 Corinthians should really be known as 2 and 4 Corinthians. Some argue that since 2 Corinthians does not have a good flow and is very choppy that maybe these lost letters were combined into the one book.
In Colossians 4:16, Paul recommends that they read a letter from the Church at Laodicea. This could possibly be a reference to another non-extant letter or it maybe referencing another book. It is possible that the book of Ephesians was not written specifically and exclusively to the Church at Ephesus, but that is another story for another time. Paul does not say the letter to Laodicea, but the letter from Laodicea. He may have just been referring to a letter that Laodicea had in their possession, possibly the one we now know as Ephesians. And while we are at it, check out Ephesians 3:3. Some think that this is another reference to a lost epistle, but more than likely it is an internal reference to Ephesians 1:9-10. You might also speculate whether or not 3 John 1:9 refers to one of John’s known writings (Gospel of John, 1 John, 2 John and Revelation) or another letter.
One more reference worth noting is Jude 1:14-15. Here Jude quotes, though not verbatim, the Book of Enoch, which is not included in the canon. Enoch, as seen in Genesis 5:18-24, is the great-grandfather of Noah and is one of the two people in the Old Testament who did not die. There are actually three books attributed to Enoch, but most scholars believe that someone else wrote them and used his name or in bigger words, it is pseudepigraphal. Jude quotes roughly from the first of the books of Enoch in Enoch 1:9 which says:
And behold! He cometh with ten thousands of His holy ones To execute judgment upon all, And to destroy all the ungodly: And to convict all flesh Of all the works of their ungodliness which they have ungodly committed, And of all the hard things which ungodly sinners have spoken against Him.
Jude is probably not unique in his usage of an outside non-Biblical source as Luke 1:1-4 implies that Luke used many outside sources to make his gospel accurate and thorough.
Some may use the missing books as an excuse to label the Bible as incomplete, but even though other things were written by Jews in Old Testament times, Jesus, who knows what’s inspired, sees fit to only quote those we have canonized. This must mean that some writings just don’t have what it takes. I wonder if God has to explain to Paul, “No offense, those other letters weren’t bad, but I’ve seen better.”
References:
- The Book of Enoch
- Are there Lost Books of the Bible?
- 1 Corinthians 5:9
- 2 Corinthians 2:3-4 ; 2 Corinthians 7:8-12
- Acts 18:1-11
- Colossians 4:16
- Ephesians 1:9-10 ; Ephesians 3:3
- 3 John 1:9
- Jude 1:14-15
- Genesis 5:18-24