American philosopher and psychologist William James gives the most incisive criticism of Christianity:

Were it true that a suddenly converted man as such is, as Edwards says, of an entirely different kind from a natural man, partaking as he does directly of Christ’s substance, there surely ought to be some exquisite class-mark, some distinctive radiance attaching even to the lowliest specimen of this genus, to which no one of us could remain insensible, and which, so far as it went, would prove him more excellent than ever the most highly gifted among mere natural men. But notoriously there is no such radiance. Converted men as a class are indistinguishable from natural men; some natural men even excel some converted men in their fruits; and no one ignorant of doctrinal theology could guess by mere every-day inspection of the “accidents” of the two groups of persons before him, that their substance differed as much as divine differs from human substance.

Source: Writings, 1902-1910, William James