Ask a Catholic

[JuliaL]

Pretty amazing that you agree a demon could cloud your mind with one breath and in the next paragraph state that you could not be deceived.  
A and not A in the same post.
Magic and logic don't go together.

You have a deficient understanding of God.

What part of ineffible don't you understand?  If God is beyond human understanding, EVERYBODY has a deficient understanding of God.

While I do not claim to be infallible nor to have a comprehensive understanding, what I can say is this:

In the Western Philosophical tradition, God is a being that is necessary (cannot fail to exist), eternal (not bound by time), immaterial (not bound be space), all-powerful, and all knowing. Most Western philosophers and theologians agree that God is all-good, or he is the perfect embodiment of the virtues of love, justice, and every other good we know. As St. Anselm of Canterbury said, God is the being "than which no greater can be thought."

Yada yada.  And because something that exists is greater than something that does not, God, as the greatest, must exist.
By that argument, the greatest anything you can mention must exist.  Look up Gaunilo's Island.
And you'd best fall back to, maximally knowing and maximally powerful because all-knowing and all-powerful are incoherent concepts.

Now, let's deal with your demons.

You suggest that I may be deceived by a demon and that if this is true, I would not even know it. Well, how is that working out? Christians believe that the greatest commandments are to love God and our neighbor. If a demon is deceiving me into thinking that I must love the Creator of all things  and to care for those around me, how is this demon advancing his own agenda which, presumably, is to have Christians do something other than those two things? IOW, if Christianity is actually a demonic plot to mislead all of its adherents, the demon who devised it has done his job too well - billions of Christians have given their lives to follow NOT the demon who sought to fool us, but the God of the myth that the demon created.

How has this demon benefited from the ruse?

And precisely who or what is the demon attempting to mislead us from? Another demon? The true God? You've not been clear on this point. If believing that Jesus is God is a deception, then what do you posit to be the truth? Who should we be worshiping instead?

Jesus said, "If Satan is divided against himself, how will his kingdom stand?" (Luke 11:18)

So, if I'm deceived by a demon into believing that there is a loving creator God who died upon the cross so that I might live with Him forever in heaven, what is the truth from which I am being diverted by this evil demon?

And what's in it for Him? We spend a lifetime doing good as much as possible...that must be unbearable for him to watch. A purely evil God would rather see evil - not good - being done. And as has been pointed out quite forcefully in the "Why Be Good?" thread, even non-believers do good to others. Wow. That must suck for our evil demon. If he is deceiving Christians by a false gospel which tells them that they must do good, then by what mechanism has the demon also deceived atheists that they must do good, also?

Are you saying that an evil God would fool Christians into believing that there is a heaven just so he can pull the rug out from under us in the end? That seems sort of anti-climatic, doesn't it? People spend their lifetimes running soup kitchens and discovering vaccines to prevent illnesses and so forth...doing all kinds of GOOD things because of their mistaken belief that "God" wants us to do them...and then he gets to say "Gotcha" when we learn that heaven is a lie after we die? Really? He has to endure watching us do good for years so that he gets a momentary thrill?

Sounds kinda silly, doesn't it? And truly beneath a Being who has that kind of power.

Bolded detail text extracted for individual treatment.

I must love the Creator of all things  and to care for those around me, how is this demon advancing his own agenda which, presumably, is to have Christians do something other than those two things?

In the hypothetical, the Creator of all things does not exist.  He was made up by the demon.
The agenda of the demon is UNKNOWN to you.  There is no reason to presume it intends anything other than exactly what happens.

then what do you posit to be the truth? Who should we be worshiping instead?

I'm not arrogant enough to claim I know truth.  You may think you are the crown of creation but you're perched on a thin film on a tiny rock. If you're all that, then God made a lot more universe than he had to.  
Why worship somebody who won't show himself? Somebody you can't trust?  Why worship anybody?

what is the truth from which I am being diverted by this evil demon?

What part of UNKNOWN do you not understand?  The demon acts by its own standards.  To you they are hidden in their entirety.

And what's in it for Him?

The victory conditions for the demon are UNKNOWN to you.

We spend a lifetime doing good as much as possible...that must be unbearable for him to watch.

Good and evil as judged by the demon are UNKNOWN to you as are its reactions to us and our circumstances.  
Perhaps the situation which results, including your delusion of infalliblity is exactly what the demon wants.

A purely evil God would rather see evil - not good - being done.

True, but in the scenario in question, there is no God.

then by what mechanism has the demon also deceived atheists that they must do good, also?

Good and evil as social norms for atheists have little to do with this scenario.  
Personally, I believe that good and evil are concepts which have allowed societies which posess them to prosper and eliminate
   societies that did not.  They do not require the existence of a transcendent being to work and so would work equally well with or without
   the demon and with or without God.
In the context of the scenario in question, the demon as a source of argument for good and evil is UNKNOWN to you.

Are you saying that an evil God would fool Christians into believing that there is a heaven just so he can pull the rug out from under us in the end?

The motivations and of the demon and the outcomes it intends are UNKNOWN to you.

and then he gets to say "Gotcha" when we learn that heaven is a lie after we die?

The point is, that when you have a putative transcendent being behind a veil of ignorance, all bets are off with respect to claims about its nature,
   intentions or actions.  Particularly if that transcendent being posesses stupendous cosmic powers beyond your comprehension.

And truly beneath a Being who has that kind of power.

Once again, you are judging the worth of a being UNKNOWN to you.  

You are consistently and inappropriately applying human expectations to an UNKNOWN being which refuses to show itself.  If it has the powers you propose, it is alien beyond comprehension.  
It must be nice to think of a sky daddy who will make all your dreams come true and mete retribution to all the baaaad people who hurt your feelings in life.