I would say that this poem is, for the most part, a very unfortunate relic of a past time when white people felt that it was completely acceptable to talk about nonwhites as if they were less than human and to look at their beliefs as if they were evil superstitions.
The major point of this poem is that black people are inherently savage and that they will never be redeemed until they can become Christian. In a sense, it is saying that the Africans need white culture and white religion in order for them to be made fully human.
This, to me, is a clear statement of the old idea of the "white man's burden." As such, it is pretty offensive to my modern sensibilities and my attitudes as a person who is not white (or at least only half white).
Interested in your question and intrigued even further by the first answer, I decided to reread "The Congo" and was struck by the verses echo from the movie Dead Poets Society:
THEN I had religion, THEN I had a vision. / I could not turn from their revel and derision. / THEN I SAW THE CONGO CREEPING THROUGH THE BLACK, / CUTTING THROUGH THE JUNGLE WITH THE GOLDEN TRACK.
They young men in The Dead Poets Society use this poem properly, I might add, using Lindsay's own "stage direction" and do great justice to this rhythmical chant: they are simply using spoken language to honor the poets of old, . . . poets of all races.
Furthermore, my take on "The Congo" generally coincides with the rest of academia that chides Lindsay for being a "well-meaning but misguided primitivist." Ah, if there were just a way to put a more positive spin on this statement! I truly believe that Lindsay was doing the African-American race a great service, giving great honor to the absolutely superior rhythm of their music and, therefore, of their poetry.
The reality is, however, that living in the turn of the century "white" world of the late 1800s tends to lend itself to decades of erroneous assumption, . . . even if he DID discover a very young Langston Hughes at a restaurant. It is incredibly important to note that Lindsay considered himself to be a staunch advocate for the African-American race! One wonders why his stress on "Their Basic Savagery" in this particular poem didn't make him read between the lines just a bit.
The irony here is, it isn't that the things in "The Congo" aren't true, per se. (Could this be the very first instance of politically-correctness?!?) It is simply the way it is presented that puts a focus more on the pre-industrialized continent of Africa (which, by the way, is not in itself a bad thing!) instead of the great advancements of the culture, especially in how it has influenced the culture of the Western world.