Question: "What are the lost books of the Bible?"
Answer: There are no “lost books” of the Bible, or books that were taken out of the Bible, or books missing from the Bible. Every book that God intended to be in the Bible is in the Bible. There are many legends and rumors of lost books of the Bible, but the books were not, in fact, lost. Rather, they were rejected. There are literally hundreds of religious books that were written in the same time period as the books of the Bible. Some of these books contain true accounts of things that actually occurred (1 Maccabees, for example). Others contain some good spiritual teaching (the Wisdom of Solomon, for example). However, these books are not inspired by God. If we read any of these books, such as the Apocryphal ones mentioned above, we have to treat them as fallible religious/historical books, not as the inspired, inerrant Word of God (2 Timothy 3:16-17).
The gospel of Thomas, for example, was a forgery written in the 3rd or 4th century A.D., claiming to have been written by the apostle Thomas. It was not written by Thomas. The early Christians almost universally rejected the gospel of Thomas as heretical. It contains many false and heretical things that Jesus supposedly said and did. None of it (or at best very little of it) is true. For example, the Gospel of Thomas has Jesus saying nonsensical things like “Blessed is the lion that a person will eat, and the lion will become human” (Saying 7), and “Every woman who makes herself male will enter into the kingdom of heaven” (Saying 114).
The gospel of Barnabas was not written by the biblical Barnabas, but by an imposter. The same can be said of the gospel of Philip, the apocalypse of Peter, etc. All of these books, and the many others like them, are pseudepigraphal, essentially meaning “ascribed to a false author.”
There is one God. The Bible has one Creator. It is one book. It has one plan of grace, recorded from initiation, through execution, to consummation. From predestination to glorification, the Bible is the story of God redeeming His chosen people for the praise of His glory. As God’s redemptive purposes and plan unfold in Scripture, the recurring themes constantly emphasized are the character of God, the judgment for sin and disobedience, the blessing for faith and obedience, the Lord and Savior and His sacrifice for sin, and the coming kingdom and glory. It is God’s intention that we know and understand these themes because our lives and eternal destinies depend upon them. It is therefore unthinkable that God would allow some of this vital information to be “lost” in any way. The Bible is complete, in order that we who read and understand it might also be “complete, and equipped for every good work” (2 Timothy 3:16-17).