How to know the Truth
Jesus said: I am the way, the truth and the life. I still stick to it. There is a point where natural revelation (what you get by observing nature) is just not enough. You need God`s revelation to understand man`s predicament, origin, objective, etc (methaphysics). This came to be in Jesus and is fairly easy to understand, so even a child can get it.
Gnosticism is the contrary, it says it takes lots of effort (as very wise men) to know the truth. It really appeals to the pride of men.
Another big mistake gnostics make is to think that truth is inside of us. It is not, it doesn't depend on us to exist. Much like the sun will keep on shinning even if you cover your eyes.