Is the Hadith a Forgery?

[robvalue]

That's Islams burden.
They just have to find a way.
Like maybe mention upcoming hadiths in the Qur'an.
If they can't then who am I to know if they are legitimate or not.

I mean, who defines what's a legitimate part of islam and what isn't?

This is the problem, yes. Anything supposedly written/inspired "by God" is a forgery, because it's not what it claims to be.

And there is no criteria. Anyone can make up any religion and say any text they feel like is "genuine". And someone else can join the religion and say that it is not genuine. There are just no standards in theology.