In Song of Myself, Whitman glorifies the self, but the self has a broader application. His "self" also applies to mankind. So, when he says:
I celebrate myself, and sing myself,
And what I assume you shall assume,
For every atom belonging to me as good belongs to you. “it is you talking just as much as myself…I act as the tongue of you”
He is also saying that along with himself, he also celebrates mankind, of whom he is a part. He also says:
“I am large, I contain multitudes.”
“For every atom belonging to me as good belongs to you.”
Transcendental in nature (Whitman was a Transcendentalist), the poem "celebrates" the divine nature of the universe, but his "self" is one with the universe.
There is an excellent discussion of the democratic and spiritual ramifications of this poem here on enotes, which you should read to understand the full meaning of this poem, however, as it says in the enotes analysis:
Heightened perception such as this also extends to other human beings, all of whom are viewed as equally divine by the persona. It is this conviction of the shared divinity of the self that enables the persona repeatedly to identify and empathize with other human beings, as in section 33: “I do not ask the wounded person how he feels, I myself become the wounded person.”
The very fact, then, that this poem celebrates the self - even if the self is part of a greater self - mankind, nature, the universe - makes this poem's focus the antithesis of conventional Christian beliefs.
Christians believe that God is the center of the universe. All created things are created by God, through God, and for God (to paraphrase the Bible). Man is God's wonderful creation, true, to be celebrated, but not to be put above God. God has created man in his own image. A Christian would consider it sinful to worship the created thing above the creator