Haruo, here's some information that I gather you probably have read which does not specifically anwer Cathy's question but makes for interesting reading anyway:

"The work for which Johnson is most remembered is God's Trombones, which caused a sensation when it was published in 1927. Impressed with the power of the imagination and speech of the Irish peasantry, Johnson wished to create a similar monument—and literary movement—for his own race. God's Trombones consists of seven sermons by a black preacher. Though Johnson did not use dialect, his free-verse paragraphs are in the rhythms of this indigenous oratory and his imagery caught the simplicity and grandeur of the preacher's imagination, nurtured on the Bible:

And now, O Lord--
When I've done drunk my last cup of sorrow—
When I've been called everything but a child of God
When I'm done travelling up the rough side of the mountain--
O--Mary's Baby—
When I start down the steep and slippery steps of death—
When this old world begins to rock beneath my feet—
Lower me to my dusty grave in peace
To wait for that great gittin' up morning.

The use of the redundant auxiliary ("done"), the biblical, concrete imagery ("cup of sorrow"), the anaphora ("When I've . . . When this . . ."), and the allusion to well-known spirituals ("Mary Had a Baby, Yes, Lord" and "In Dat Great Gittin' up Mornin'") are typical of Johnson's style in this work. In an actual church sermon the last line would be a signal to the congregation to break into singing the spiritual."
http://www.english.uiuc.edu/maps/poets/g_l/johnson/creation.htm
-snip-

"[James Weldon] Johnson's most famous literary effort was God's Trombones published in 1927. God's Trombones are a collection of poetic sermons written in free verse. It is said the Johnson considered the voice of the black preacher to be a musical instrument "not a piano... or trumpet but a trombone"
http://www.nathanielturner.com/godstrombones.htm

-snip-

Also, see the words to The Creation" (from God's Trombones, 1927) along with the editor's note [without accompanying references] to Johnson being an agnostic:

"5. How can one justify the author's use of the compensatory Christian ethic in "Lift Every Voice," "Bards," "Listen Lord"--a prayer--and the sermons in Trombones, when he himself is an agnostic? (Refer to outside sources for the latter two poems.)

http://web.csustan.edu/english/reuben/pal/chap9/jwjohnson.html