It's the easiest way to launch into a "shadow" story, wherein the traits that we've held down all our lives turn into something evil. If we can integrate the things we're afraid of in ourselves, we can redeem them and they make us stronger and better.
"If you bring forth that which is within you, what you bring forth will save you. If you do not bring forth that which is within you, what you do not bring forth will destroy you."
I liked better the way the original Star Trek provided for that, when they split Kirk in two with the transporter.
ST-TNG did a poor imitation of that excellent show by splitting Will (Number One), showing how the same person would develop different personalities based solely on environment.
Good quote. But we should note the Gospel of Thomas didn’t make it into the canon.