What belief is implied but not explicitly stated in this excerpt?

And if such men can boast of greater degrees of knowledge than any African is entitled to, I shall let them enjoy all the advantages of it unenvied, as I fear it consists only in a greater share of infidelity, and that of a blacker kind than only skin deep. And if their complexion be not what I may suppose, it is at least the nearest in resemblance to an infernal hue.

the others had been answered twice with no comment box available. So here is my question. Why is C. correct on edge?