Our number one enemy as Christians is Satan, and he is everywhere in today's world. The enemy shows its face in the form of sins, temptations, and virtually any other area where you can be led astray. Whenever you feel the urge to lie, cheat, steal and commit other immoral acts - that is Satan at work. You must do your best to avoid these acts. By giving into them, you are letting Satan win.
In your life, if you come across those you do not get along with and you consider them your earthly "enemies," understand that they are just humans, too. They may be bad people, but as Christians we cannot let ourselves become dark and malicious like other people. Jesus taught us to NOT seek vengeance. If anything, we should seek to understand and forgive our enemies.
But in the end… always remember: Satan is Christianity's enemy. Don't let the enemy win - use the Bible to stay on course and defeat Satan. Please read these Bible verses on enemies if you need help understanding.