Well done for identifying how these two characters act as foils. Let us remember that foils are characters who are deliberately set against each other because of their differences, and, as a result of being a foil, their characteristics are made to seem more extreme because of the comparison. In a sense, both Allie and Spellgood have come to the jungle with the same idea: to save the natives, and both are incredibly zealous about achieving their objective. However, it is clear that Spellgood represents the religious equivalent of Allie.
The central differences between them are what they believe will save the natives. Whilst Spellgood believes in the Bible for the natives' salvation, Allie is harshly critical of such beliefs and places his faith in technology and science. Both are somewhat megalomaniac characters, and Spellgood is somebody who excites disgust in the reader, as he is shown to be manipulating the natives to gain numbers rather than converts. His incorporation of new technology into his ministry, for examply sending videotapes of his sermons to groups of Indians when he cannot preach in person, is shown to be a way of building his own empire and maintaining control rather than a sincere desire to help the Indians. Both are trying to build their own empire in the jungle, but using different methods.