Not to be a stickler but that is a weak argument, even though your overall point is in response to your own straw man.
The plan of salvation is for mankind. The demons can know the plan but would never be given the Divine Sovereignty of God to move them to repentance and salvation. Demons cannot be justified. Mankind can be. Demons have their fate sealed already. They 'believe' the facts of their eventual eternal punishment at the Hands of Christ and know they can do nothing about it but deceive as many souls as possible to join their torment. This is the difference between an acknowledging 'belief' and the much different God ordained saving faith/trust.
Jesus is the object of judgement for demons. They believe that and know that to be true.
Jesus is the object of salvation for those who trust in His Work and Resurrection. For those who reject, their fate will be the same as the demons.
Whoa!
Not such a weak argument that the apostles wouldn't use it.
James 2:19-21
19.Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
20.But wilt thou know, O vain man, that faith without works is dead?
21.Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?