More directly... to follow. The demons know who God is but refuse to follow and worship. We are not merely called to believe as a knowing but to believe as a following all He has commanded... and that is a doing.
And when we fail to do as He commanded, because of our saving faith He is faithful and just to forgive us our confessed sin. We confess our sin to Him and He is Faith full. If we were not already saved when we confess to Him, the process of forgiveness would require getting saved again so our faith would be connected to His Spirit in us.
IOW: We confess to Him, because we have saving faith IN His Promises.