Fear of consequences does not produce love or even obedience. It is ONLY an enduring, reverential, and respectful love relationship with God that will keep us from sin. When we reverence God’s Holiness, when we respect His way, when we have a genuine relationship with Him we will find it increasingly uncomfortable to disobey Him, we will avoid sin.
If the people of Capernaum refused to believe Jesus when He identified Himself as the Messiah, do you think they would have believed it if the devils declared it?? Certainly not, it would just have been a bigger spectacle. Likewise, anyone who refuses to accept Jesus as Saviour now, will reject Him even if demons jumped up and declared him to be the Son of God.