That's a good question, and its why I've been studying the gnostics and their texts for a few years now. Still a newb, though.
I think the gnostic texts go a long way to verifying the fact that a "Jesus event" occurred. Something profound happened and mind-fucked a lot of people all at once.
The part of the NT that I know is reliable is the Gospels. Jesus often speaks symbolically, but he is always logical. There's sense in what he says.
That may not seem like much, but this is a very barbaric world, and the barbarism comes from people being delusional. For example, something like half of all indigenous tribes in the New World were in some way cannibalistic. In one pyramid in Mexico city, there was a wall made of over 30k human skulls, and priests sacrificed and ate their hearts because they believed the gods required it or they would destroy the world. This is not an exaggeration. Contrast that with Jesus' words, "the truth will set you free," and "love your neighbor as yourself." My point is, its easy to be a modern person and think people are rational because of the world that Christianity made. Not perfectly... There were still power games and atrocities... But this rational and mostly safe world is the direct product of Christian thinking, which demanded that people be grounded in reality or at least trying to be. Because the truth will set you free. Illusion is enslavement in itself, and leads to horrible outcomes, like human sacrifice or the brutal Roman empire or modern fiat enslavement.
And that's also the source of my frustration with modern Christianity. Its falling back into illusion. They refuse to be reasoned with. They spin their wheels trying to justify one part of the Bible that they don't understand with another part that they don't understand, and get angry when you have simple and non-hateful explanations that are well supported by the bible but which they didn't hear from their church, so they won't even be nice, much less think about it. Enslaved by illusion. Not following Jesus.
All you gotta do is think. It all fits together, it all makes sense. And if an explanation doesn't make sense, **_then its wrong._** And I know Jesus agrees because he said as much, several times.