Can we know if a message is from God?

Many people have claimed and continue to claim to have all sorts messages from God, but are they justified in making these claims? Let's say one takes the existence of spiritual or non-physical beings seriously and avoids making simplistic assumptions about them. Given this starting point, what can we say about messages that purport to be from a particular non-physical being such as God?

I will start by considering how we know the sources of messages in the physical world. If you claim you have a message from your friend, then you are claiming that the message is not from anyone else. But how can you be sure someone else didn't send it to you? Here are four general methods:

1. You have observational evidence. For example, your friend gave it to you in person or confirmed it by talking on the phone. You know it is him since you can discriminate his appearance or voice characteristics from those of other people.
   
2. The letter has his signature on it. This can confirm that he wrote it if you can discriminate his signature from other people's signatures and also from attempts at forging it.
   
3. The letter is written in his style that no one else has and that cannot easily be imitated.
   
4. The content of the letter contains knowledge that only he (and possibly you) has. You need to know that only he has this knowledge.

So, do these methods apply to knowing if a message is from God? Let's assume that non-physical beings exist (somehow) and can communicate (somehow) with the physical world. Indeed, Christianity has a plethora of these non-physical beings, sometimes called spirits, including all sorts of angels, demons, and gods (if one takes the Old Testament literally). But a priori we can't restrict ourselves to only the non-physical beings in the Christian Bible, perhaps there are innumerable others of all kinds and abilities. To avoid having the name of the being distract our analysis let us denote the set of non-physical beings as G = {G_0, G_1, G_2, ..., G_n}, where G_0 denotes the Christian God, and G_i(i>0) denotes the other non-physical beings, and n is large and could be infinite. Now say you get a message that purports to be from God, that is G_0. How can you tell it is from G_0 and not G_i(i>0)? Can the methods listed above enable you to tell if it is?

1. The problem with the first method is that non-physical beings don't have any particular appearance or voice characteristics. At most one sees or hears a vision. But what is to distinguish a vision provided by G_0 from a vision from some other non-physical being, G_i(i>0)? Presumably there are other non-physical beings that could create just as good visions that also claim to be from God even though they are from some other G_i. It seems that appearance is not going to work to tell if it is from God.
2. What about some sort of God-signature in the message? This is probably the most common means of justifying a message from God, and the signature most Christians accept is a "work of power", or more simply a miracle. But is a miracle sufficient as a God-signature? First there is the problem of showing that it actually is a miracle in some way caused by a non-physical being, but let's assume this can be done. But is it only God that does miracles, of the potentially innumerable other non-physical entities? Surely there could be many G_i(i>0) beings that can do powerful miracles. Ah, the trick that is used here is to assume that everything has one of two sources: either God or the devil. So then once one has a miracle one has only a two-class problem: either it is a God-signature or a devil-signature, and this is potentially feasible to determine if the devil is not very intelligent. But why only two sources and why dismiss all the other sources, G_i(i>0)? This dichotomization is a masterful sleight-of-hand. It is saying that none of the other spiritual beings are intelligent or independent enought to operate on their own or to fool humans. That is definitely not taking the non-physical realm seriously, and can be thrown out as an unjustified assumption. Another objection (from the Calvinists) is to claim that every being acts at the direction of God, and God would not want us to be deceived and so he wouldn't let us be confused by messages from many other G_i's. But even if one accepts the premise that all beings act at God's direction, the conclusion is surely false as it would imply that no human is going to deceive you either. So we are stuck with not having any signature to determine if the message is from God.
3. What about style as a way to identify the sender? It can work to some extent for distinguishing human messages, as we know the range of styles people have and can sometimes identify unusual aspects of a particular person's style. But that's the rub; what styles do other non-physical beings have in their communications? How can we say a particular style is God's style and not the style of some other G_i? Christian teaching in this focuses on the dichotomized problem: distinguishing God's message from Satan's, but as I argued above this is a false dichotomy as it excludes most of the problem. Alternatively, one claims that it is from God if it is compatible with previous revelation (namely the Bible). But this fails for two reasons: we need to know that the previous revelation was from God which assumes we have solved the problem, and also it assumes the non-physical beings aren't intelligent enough to fool us with messages in the style of previous revelations. Thus style is not going to be a useful discriminant for knowing if the message is from God.
4. What about the fourth method: the message contains some knowledge that only God has? The classic example of this is future predictions or prophecies. There are a host of problems with prophecies including: showing that they occurred before the event (as much prophetic literature was actually written after the event but slyly claimed to be written before), and showing that the prediction isn't simply a good-guess or lucky fluke, and showing that the prophecy isn't so vague or general so as to have many fulfillments. And indeed all the prophecies I have seen in the Bible suffer from one or more of these problems. But let's assume we can get a genuine future prediction that comes true and wasn't a fluke. Is that a sufficient way to know that whatever message came with it is from God? Well, if non-physical beings are not constrained by space they won't be constrained by time either. Space and time are part of the same physical reality "space-time" that governs physical things, but not non-physical things. So presumably there are other non-physical beings, G_i(i>0), that have access to events anywhere and at any time and so could also be the source of future predictions or prophecies.

My conclusion is that the only way we can be confident in the source of messages from non-physical beings is to assume that they are all, apart from God, less intelligent than humans and so unable to fool us. However, if one really believes in non-physical beings, then one should surely take them seriously and acknowledge that there may be many of them, that they may be far more intelligent than humans, and that they may intentionally deceive us. As I have argued here, none of our usual method for identifying sources of messages works with intelligent non-physical beings. Thus since our knowledge of them is limited to what they choose to tell us, and who they say they are may not be true, we have no way of identifying the actual source (God or some other G_i) of any particular message.