That's quite an ambitious claim.It is illusion because the existence is a dream that is created by the God=Consciousness=Source. Again I am not talking about an angry man in the sky. God isn't an external thing like Santa Claus. It is your consciousness so it is you.
How is it proven true?
More ambitious claims.We are creating/dreaming the universe within our consciousness but we have forgotten our true nature and now we are suffering (the Original Sin). In a way we got trapped in our own monstrous creation away from the God's glory.
Now things may be going well in one's life and when that happens one tends to think itself as strong and as some kind of genius but think about dying and getting reincarnated as a disabled person.
So in order to end suffering we have to connect with the source again. Although on the surface enjoying the material world is considered "healthy" if you got lost in the illusion you will get devoured by it.
Evidence?