One method to decide what book should or should not be in the Bible is to only consider those that were written by the Apostles themselves.They heard directly from Jesus, so we should trust what they say as scripture. Problems arise when you consider the Gospels of Mark or Luke. Neither Mark of Luke were apostles, but we consider what they write to be scripture based on their close ties to the Apostles. Mark to Peter and Luke to Paul. Problem solved. But not really, as this creates more questions as well.
Clement wrote his letter to the Corinthians around 80 AD and had close ties to the Apostles, much like Mark and Luke. In fact, this Clement is likely the man Paul is referring to as a “fellow worker” in Philippians 4:3. In Clement’s letters, he writes with the assumption that Christians across the Mediterranean would recognize his authority. His letters were treasured and circulated just as Paul’s letters were. Many early Christians did consider Clement’s letters as scripture and in the Coptic churches it is considered scripture to this day.
Other questions remain. How do we know who wrote the books in the Bible? The obvious answer would be, “Well, they say who wrote it right in the beginning! Paul to the… or Peter to the…” While that is true, there are many books that make those same claims, that are not in the Bible. The Gospel of Thomas claims to be written by Thomas and the Protoevangelium of James claims to be written by James, but these are known to written by impostors trying to increase the authority of what is written by associating it with an Apostle. When you get into the Gospels, the water gets even murkier. None of the Gospels say who wrote them. They didn’t come with the title, “The Gospel of …” An outsider picking up the Gospel would have no idea who wrote them. And what of Hebrews? Though there are many debates regarding who the author is, the author remains unknown, yet it too also made the cut.
So how does the average person know who is an imposter and who is authentic? We know who wrote which Gospel based on the testimony of the Church.The church in the second and third centuries examined the various works and decided which was legitimate and which were not based on the testimony of the faithful. It is from the authority of the church in these later centuries that we have a Bible with Gospel of Mark in it and not the Gospel of Thomas.
Scripture itself can not tell someone what is considered scripture, there must be an outside source.