Calvinists like to say that if you have free will, especially if that is to the degree that you can reject salvation, then God is not sovereign. Scripture tells a very different story. We frequently see that God has, in His Sovereign capacity as Lord, created beings who are more than capable of disobedience. This is an ability we frequently display, to our OWN destruction. God is absolutely sovereign. Calvinists have no monopoly on knowing that fact. 

It is just that His sovereignty does not preclude our responsibility. Rather, because that is how He has decided things are, it is an expression of it.