Walter "Walt" Whitman (May 31, 1819 – March 26, 1892) was an American poet, essayist and journalist. A humanist, he was a part of the transition between transcendentalism and realism, incorporating both views in his works. Whitman is among the most influential poets in the American canon, often called the father of free verse. His work was very controversial in its time, particularly his poetry collection Leaves of Grass, which was described as obscene for its overt sexuality.
Whitman was deeply influenced by deism. He denied any one faith was more important than another, and embraced all religions equally. In "Song of Myself", he gave an inventory of major religions and indicated he respected and accepted all of them—a sentiment he further emphasized in his poem "With Antecedents", affirming: "I adopt each theory, myth, god, and demi-god, / I see that the old accounts, bibles, genealogies, are true, without exception". In 1874, he was invited to write a poem about the Spiritualism movement, to which he responded, "It seems to me nearly altogether a poor, cheap, crude humbug." Whitman was a religious skeptic: though he accepted all churches, he believed in none. God, to Whitman, was both immanent and transcendent and the human soul was immortal and in a state of progressive development. from wikipedia
Wikipedia lists Walt Whitman as an humanist. He has been referred to as an atheist, and below are some of the quotes used to support that position.
"I like the scientific spirit - the holding off, the being sure but not too sure, the willingness to surrender ideas when the evidence is against them: this is ultimately fine - it always keeps the way beyond open." - Walt Whitman
"God is a mean-spirited, pugnacious bully bent on revenge against His children for failing to live up to his impossible standards." - Walt Whitman
"Pointing to another world will never stop vice among us; shedding light over this world can alone help us." - Walt Whitman 
“The love, the life of their bodies, meaning and being,”
Curious here behold my resurrection after slumber,
The revolving cycles in their wide sweep having brought me again,
Amorous, mature, all beautiful to me, all wondrous,
My limbs and the quivering fire that ever plays through them, for
reasons, most wondrous,”
To The Garden by Walt Whitman