There are four gospels in the New Testament. That fact was pretty much established in the first century: The gospels which ancient Christians assumed were written by Matthew, Mark, Luke, and John, are considered the canonical gospels. They’re the valid ones. Others, not so much.
For good reason. If you’ve ever read those other gospels, they read like obvious Christian fanfiction. They get Jesus all wrong. He’s less patient, more angry or judgey, more legalistic or more libertarian, or the author puts words in his mouth which are just plain heretic. It’s kinda obvious why the ancient Christians didn’t put ’em in their bibles.
The Gospel of Thomas is much less obvious. Yeah, but it’s because we don’t understand them that we can’t just definitively say, “Oh, it’s heretic.” It might be heretic, and some of the ancient Christians, like Eusebius of Caesarea and Origen of Alexandria, were entirely sure it was. There are some sayings in there which are kinda weird, which we don’t entirely understand, possibly because the gospel was composed by gnostics. (No, the apostle Thomas didn’t write it. That, we’re sure of.) Gnostic writings weren’t meant to be understood, unless you paid gnostic teachers to decode ’em for you. Whereas the canonical gospels are meant for everybody to understand.
Thomas is what scholars call a logia, a list of sayings. It’s what we suspect Matthew and Luke used as one of their sources for their gospels: One or more logias, plus Mark. As a logia, it only consists of Jesus’s sayings—not his acts, miracles, birth, death, resurrection, nor any of the longer and more complex teachings. Thomas doesn’t include the context of these sayings either, which makes them a lot harder to interpret if this is the only gospel you have. It definitely has its deficiencies. That’s why Christian scholars might read it, but none of us but the crackpots seriously think about adding it to the bible.
Thing is, Thomas overlaps the canonical gospels an awful lot. Read it for yourself: Most of the sayings are also found in the New Testament. And in some cases, with an extra sentence or word, which puts a whole new spin on their interpretation. But no, this doesn’t mean Thomas explains what the bible really means. Let’s not go the crackpot route, okay?
Our current copy was missing for centuries till 13 books, written in Coptic, were discovered in a sealed jar near Nag Hammadi, Egypt, in 1945. (They’re now at the Coptic Museum in Cairo.) Thomas was included in Codex 2. After their discovery, scholars soon realized three of the Oxyrhynchus Papyri (discovered in Oxyrhynchus, Egypt, in the late 1800s and early 1900s, and now in museums round the world) were parts of Thomas written in Greek.
My translation below comes from the Coptic text. Some scholars split Thomas into chapters and verses. I didn’t do that; each saying (or logion) has a number, and each “verse” is a letter if you wanna refer to that specific part of the saying. And no, I’m not providing commentary; it’s not bible.
So if you wanna read it for yourself, here you go.