If you assert that, then I must accept you believe that. Therefore, what is the sufficient cause of God?
Gotcha! Is that what you thought? Did you really think you had posed the ultimate unanswerable objection to the existence of God?
I unfortunately don’t have time to try to go through all the responses to this old canard. Plus, I’m not a philosopher - here’s a site for starters that does an adequate job. There are probably a hundred others:
http://www.godandscience.org/apologetics/who_created_god.html