We should never get so complacent in our security in the Lord that we forget that we DO have an enemy who desires to destroy us.
1 Peter 5:8: "Be sober, be vigilant, because your adversary the devil, as a roaring lion, walks about, seeking whom he may devour."
Whether or not he can get your soul, he can definitely put thoughts in your mind that shouldn't be there and convince you that they are from God if you let your guard down. He also knows your weaknesses and any sinful desires you might have, and he will use those against you.