Those who believe that god is speaking to them are likely suffering from auditory hallucinations. Here is an interesting article from Psychology Today on the subject:
Are You Hearing Voices?
"Methought I heard a voice cry, 'Sleep no more!'" -- Macbeth, Act 2, Scene 3 Published on September 14, 2009 by Jerome Litt, M.D. in Odd, Curious, and Rare
Auditory hallucinations, the false or distorted perceptions of sound, music, noises, or voices “are one of the most awe-inspiring, terrifying, and ill-understood tricks the human psyche is capable of." The voices may tell the person to perform an act (commonly referred to as a "command hallucination"). The man who killed a Swedish politician in September 2003 told the police that voices in his head told him "to attack."
Auditory hallucinations are most often a symptom of severe, disabling psychiatric or neurological illness. It affects as many as 70 percent of those with schizophrenia and about 15 percent of those with mood disorders such as depression or mania. Studies of combat veterans diagnosed with post-traumatic stress disorder (PTSD) have found that 50–65% have experienced auditory hallucinations.
Some types of auditory hallucinations include musical hallucinations, where people will hear music playing in their mind, usually well-known songs they are familiar with. This can be caused by lesions on the brain, occurring most often from strokes, but also tumors, encephalitis, or abscesses.
It is also possible to experience musical hallucinations from listening to music for long periods of time. Musical hallucinations are most common in older people, especially women, with hearing loss. Beethoven had musical hallucinations after he became deaf. Fortunately he continued to compose . . .
Many historical figures, from Socrates to Caesar to Descartes, have been reported to "hear voices." Hallucinated voices are also known to occur during states of religious or creative inspiration. Joan of Arc described hearing the voices of saints telling her to free her country from the English. Rainer Maria Rilke heard the voice of a "terrible angel" amidst the sound of a crashing sea after living alone in a castle on the Adriatic for two months. This experience prompted his writing the poem, Duino Elegies.
Absinthe, a bitter spirit containing wormwood, was one of the most popular alcoholic beverages of late 19th century Europe. The emerald green drink—"the green fairy"—was consumed by people from all walks of life. Side effects from the active ingredient—thujone—include hallucinations, seizures, and death. The most memorable celebrity known as an absinthe drinker was Vincent van Gogh. Henri de Toulouse-Lautrec, Paul Gauguin, Oscar Wilde and Edgar Allan Poe were all passionate about absinthe.
The Transformers star, Megan Fox, admits she has struggled with mental health problems since her youth, and, although she hasn't been officially diagnosed, she is convinced she shows symptoms of a serious psychiatric condition which often torments her with auditory hallucinations, paranoid delusions and social dysfunction.
The following medications have been reported to give rise to auditory hallucinations: Ambien, Amphetamines, Catapres, Celontin, Darvocet, Darvon, Demerol, Ketalar, Khat, Neo-Synephrine, Persantine, Prialt, Provigil, Symmetrel, Ultram, Tenuate, Valtrex, Vioxx, Visken