A Comparison with the ether
The ether is a medium that was once supposed to fill all space and to support the propagation of electromagnetic waves. This 'ether' has always been around us, but it is only since recent centuries that we begin to understand how we can communicate over distance using electromagnetic waves. The last century has seen enorm advances in communication technologies using radio waves.
Through the centuries there has always been the notion that beyond the physical world in which we live, there is also a spiritual world. This spiritual world is suposed to be all around us filling up all space much in the way as we thought the ether would do. We don't know however, how we can communicate effectively with this spiritual world. It might be that this spiritual world is just the space between a sender and a receiver. Both could be physical persons that exist in different dimensions or times and the only thing in common might be this medium, this 'spiritual world'.
Many cultures have rituals you need to perform in order to establish contact with the unknown, mysterious, powerful entities, which surround us but whose existence lies beyond our normal perception.
Some of these rituals are intended to please God or other Deities and others are meant to communicate with them directly. It seems that the source contacted depends on the intention of the diciple that performs the ritual, but the methods used in different religions are remarkable similar. Praying, meditation and séances all could be meant to open a communication channel in the spiritual medium allowing for some kind of direct communication with God or any other non-physical entities, or even with real people who exist in other dimensions or times.
Some theories explain that meditation, trance and other rites might allow for a temporarily disconnection of the human soul from the body after which the soul can briefly visit the spirit world and communicate with other souls, ghosts and spirits.
Nostradamus studied ancient books about occult art. These books often describe methods of one-way and two-way communication with 'who knows what'.
The idea of such a communication channel might be far fetched and the existence of a spirit world as a medium that can be accessed by using rituals has never been proved, still many religions use a form of praying or meditation to contact their God directly. Many followers of religions are convinced that by directing their prayers to god, he will somehow receive their petitions.
In some occasions angels have been mentioned as intermediate for the communication between God and the disciple. Apparently these spiritual beings have visited disciples in their sleep relaying messages from God or other godlike entities. An angel or spirit is a being that has no body. In general it can't be seen or alternatively only one person can see it, which normally is the person that receives the message. Whether prophets have seen an apparition or not they often claim to have heard a voice that only they could hear.
It seems from the above that communication with God or any other godlike entity through meditation or praying resembles hearing voices in your head. This could be very effective but at the same time very difficult to proof and even more difficult to get accepted by others.
Voices in your head is, generally an indication of a medical problem, not an indication of a message from another dimension.
In order to be able to convince others that such a communication really took place, the message must be very special. The emphasis therefore is on the contents of the message. A prophet must be able to prove beyond reasonable doubt that the message could not have been conceived in any other way than what he says.
True predictions are very strong arguments to convince people that the message must be from super natural origin. If some of the messages are proven to be true predictions of the future then the receiver might be accepted as a prophet. Further messages can then be accepted as true prophecy coming from the source that the prophet indicates.