Jesus tells us that evil thoughts defile us.
"For out of the heart come evil thoughts, murders... false witness, slanders. These are the things which defile a man."
Matthew 15: 19
John Wesley believed that a Christian can be free from evil thoughts. Does it then follow that a follower of Jesus will never have a wrong thought? I believe that the distinction is between a thought of doing evil and an evil thought. When Jesus was tempted in the wilderness He had to grasp in His mind the possibility of doing evil. But there is a distinct moment when a thought to do evil becomes an evil thought. This point in time is when the idea ceases to be a choice or a fleeting notion and becomes an imagining in the heart. As long as I struggle against it, shutting it out every time it comes into my mind, it remains a thought of doing evil. But the instant I give it entrance and begin to entertain the possibility (imaginings) it becomes an evil thought.
The most subtle danger is the idea that our thoughts are our own and harm no one, not even ourselves; that to explore the possibility of sinning is not really sin; that to think unjust thoughts is not slander; that to wish harm is not murder or violence; that to imagine and fabricate incidents (daydreaming) is not false witnessing and lying.
In the secret place where no one but ourselves can enter, we become more evil or more pure by the thoughts we entertain. The choice is ours.