How can I overcome my evil thoughts even though I am Christian already?

Answer:

I've always been amazed at how many people think that Satan gives up and goes away just because a person has become a Christian. Satan isn't going to stop tempting you with sin (evil thoughts), he is going to work all that much harder to bring you back into his camp. "Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour. Resist him, steadfast in the faith, knowing that the same sufferings are experienced by your brotherhood in the world" (I Peter 5:8-9).

In order to tempt you with sin, Satan has to get you to think about the sin. If you never thought about the sin, then Satan would be unable to tempt you or anyone else. Where thoughts of sin become wrong is when you seriously consider them as something you would like to do, but you just haven't had the opportunity. Such thoughts are lusts. For example, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28). This is not simply seeing a woman or having a passing thought that she is attractive, but entertaining thoughts of committing adultery with a woman. Such thoughts are sinful.

You battle Satan by resisting the urge to think and pursue the things you know are wrong. That is what God asks of you.