The worst enemy is one who can endanger your soul. So who is he? Satan? No, because he is powerless if you remain humble in God. Someone in the world? Well, you may have many of them all fighting you in various ways, but they cannot harm your soul if you don't do anything wrong or bad. In reality, your worst enemy is yourself, since only you can choose to stay away from God or to obey or follow him. It's also your choice to let Satan or others influence you or not. Conquer yourself and you'll have conquered all your enemies with God becoming your friend.