We are His, even when damaged. Did you know that exorcists say one cannot 'sell their soul' to the devil, just get tricked into thinking they did? Because our souls don't belong to us, they are His.

Now, they definitely can mess with our perceptions and our brains to lead us into pushing ourselves away from God. They can physically damage our brains to retard the areas most used for contemplating or understanding anything outside of or above the plain physical world. They can damage and destroy but they can never create. Humans can partake in creation, though, through God's blessing, and this is one thing that the demons cannot stand.