Christ was both fully human and fully God--another seeming contradiction, but with God, all things are possible.
Don't forget that Christ taught free will through the parable of the Prodigal Son. Free will is assumed and mentioned more times in the Scriptures than not (The whole command to repent is based on the concept of free will). So we must believe that it exists. How we reconcile that with the teachings from Romans about predestination can get quite tricky.
I'm not saying predestination doesn't exist, I'm saying that it doesn't exist in the form that monergists want to say it is.