quote""In James 2:19, we see that the demons believe "mental assent" that "there is one God" but they do not believe in/have faith in/trust in/reliance in Jesus Christ for salvation (which is what it means to believe in Christ unto salvation).::
you quoted from James 2:12-26 which says that faith without works is dead; Interesting.
in essence if you are a believer and you do the father's will you will be saved
Faith Without Works Is Dead
Jas 2:14 What good is it, my brothers, if someone claims to have faith, but has no deeds? Can such faith save him?
Jas 2:15 Suppose a brother or sister is without clothes and daily food.
Jas 2:16 If one of you tells him, “Go in peace; stay warm and well fed,” but does not provide for his physical needs, what good is that?
Jas 2:17 So too, faith by itself, if it does not result in action, is dead.
Jas 2:18 But someone will say, “You have faith and I have deeds.” Show me your faith without deeds, and I will show you my faith by my deeds.
Jas 2:19 You believe that God is one. Good for you! Even the demons believe that—and shudder.
Jas 2:20 O foolish man, do you want evidence that faith without deeds is worthless?
Jas 2:21 Was not our father Abraham justified by what he did when he offered his son Isaac on the altar?
Jas 2:22 You see that his faith was working with his actions, and his faith was perfected by what he did.
Jas 2:23 And the Scripture was fulfilled that says, “Abraham believed God, and it was credited to him as righteousness,” and he was called a friend of God.
Jas 2:24 As you can see, a man is justified by his deeds and not by faith alone.
Jas 2:25 In the same way, was not even Rahab the prostitute justified by her actions when she welcomed the spies and sent them off on another route?
Jas 2:26 As the body without the spirit is dead, so faith without deeds is dead.
Yes demons believe in Jesus and God but will not do the father's will as they rebelled. just like you mentioned, this is true they have no love for him. Because they Hate God they will not obey and do his will. the point I want to make is that simple belief is not enough, action and obedience to Christ's teachings are also essential, they demonstrate that you love Jesus and the father who sent him.
see the parable below from Jesus;
The Parable of the Two Sons
Mat 21:28 But what do you think? There was a man who had two sons. He went to the first one and said, ‘Son, go and work today in the vineyard.’
Mat 21:29 ‘I will not,’ he replied. But later he changed his mind and went.
Mat 21:30 Then the man went to the second son and told him the same thing. ‘I will, sir,’ he said. But he did not go.
Mat 21:31 Which of the two did the will of his father?” “The first,” they answered. Jesus said to them, “Truly I tell you, the tax collectors and prostitutes are entering the kingdom of God before you.
Mat 21:32 For John came to you in a righteous way and you did not believe him, but the tax collectors and prostitutes did. And even after you saw this, you did not repent and believe him.
Simple belief is not enough, you must show it;
Salt and Light
Mat 5:13 You are the salt of the earth. But if the salt loses its savor, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled by men.
Mat 5:14 You are the light of the world. A city on a hill cannot be hidden.
Mat 5:15 Neither do people light a lamp and put it under a basket. Instead, they set it on a stand, and it gives light to everyone in the house.
Mat 5:16 In the same way, let your light shine before men, that they may see your good deeds and glorify your Father in heaven.
I have more to say and will do so in other posts,
Blessings.