Lovecraft's guiding literary principle was what he termed "cosmicism" or "cosmic horror", the idea that life is incomprehensible to human minds and that the universe is fundamentally alien. Those who genuinely reason, like his protagonists, gamble with sanity. As early as the 1940s, Lovecraft's work had developed a cult following for his Cthulhu Mythos, a series of loosely interconnected fiction featuring a pantheon of humanity-nullifying entities, as well as the Necronomicon, a fictional grimoire of magical rites and forbidden lore. His works were deeply pessimistic and cynical, challenging the values of the Enlightenment, Romanticism, Humanism and Christianity. Lovecraft's protagonists usually achieve the antithesis of traditional gnosis and mysticism by momentarily glimpsing the horror of ultimate reality and the abyss.
Although Lovecraft's readership was limited during his life, his reputation has grown over the decades, and he is now regarded as one of the most influential horror writers of the 20th century. According to Joyce Carol Oates, Lovecraft — as with Edgar Allan Poe in the 19th century — has exerted "an incalculable influence on succeeding generations of writers of horror fiction". Stephen King called Lovecraft "the twentieth century's greatest practitioner of the classic horror tale." King has even made it clear in his semi-autobiographical non-fiction book Danse Macabre that Lovecraft was responsible for his own fascination with horror and the macabre, and was the single largest figure to influence his fiction writing.
Lovecraft's themes and ideas have had a profound effect on culture and literature in general, and have embedded themselves into the foundation that is used for horror associated with the strange.