Believers have faith. The demons have knowledge. There's a big difference. They had their chance and threw it away. They turned their backs on the redeemer and there's no turning back. They're like people who have fallen away. There's no going back.
Heb 6:4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.
The falling away in those verses is referring to Jews who have heard the gospel and rejected it going back to Jewish ways of animal sacrifice. It is NOT speaking about Christians who do wrong after salvation.
