In existentialism philosophy that is includes faithfulness and atheism, human argument has a special position. In other hand various viewpoints in God subject give special attractiveness to this philosophy. So regarding to faithful fundamental thinks of Kierkegaard and in its front point regarding to atheism methods of Sartre as two effective thinkers of in contemporary philosophical thinks, and in specific case, existentialism, in this paper it had tried that human authenticity and its relation to God existence, be study and analyzed. So, regarding to places of both philosophers about God, authentically factors in human had been studied and relations between human kinds and distinct them from each other, or in other words social and individual places of human also been studied and were answered in forms of different questions. For example, Kierkegaard introduce true faithful to God as the key of human authenticity, in which the human individuality is in before the God. And to Sartre, the bad faith and trooping in existence in being in it, instead of human authenticity to being for itself is authenticity factor to human, and for him, God faithful can had an effective rule in human authenticity.