1 Peter 5:8-9

Be sober (well balance and self-disciplined), be alert and cautious at all times. That enemy of yours, the devil, prowls around like a roaring lion, seeking someone to devour. But resist him, be firm in your faith (against his attack-rooted, established, immovable), knowing that the same experiences of suffering are being experienced by your brothers and sisters throughout the world. 

The devil will talk to us and suggest things. He is sly about it and slips it in when the guard is down. We are not to accept every thought that comes into our head but cast down imaginations that are not Godly. We are to resist the devil and speak the Word; the Bible says the devil will flee. Be strong in the Lord, and keep your guard up.