Arthur Schopenhauer, in The Horrors and Absurdities of Religion
Another in the Penguin Great Thoughts series, and I think Schopenhauer nails the way in which contemporary strong humanists (most of whom are vociferous atheists) owe more to Christian theology than they might like to think (see my post on Lewis Wolpert).
I'm also reading the William James collection, On a Certain Blindness in Human Beings, and apart from the shared concern for animals, they couldn't be more different. Schopenhauer negative about life (he adopted the worst interpretation of the Buddhist preoccupation with suffering, a "life will always be shitty" fatalism), James positive (you could interpret parts of his philosophy as an early form of cognitive behavioural therapy to treat his own depression). Schopenhauer bracingly right about religion ("The Rationalists say to the supranaturalists: 'Your doctrine isn't true.' The latter retort: 'Your doctrine isn't Christianity.' Both are right."), James still wanting to make the leap of faith in the absence of proof, and remain a Christian while abandoning historical Christianity. James wants to believe, but I'm never quite sure what he left himself to believe in.
Old fashioned theism was bad enough, with its notion of God as an exalted monarch, made up of a lot of unintelligible or preposterous 'attributes'; but, so long as it held strongly by the argument from design, it kept some touch with concrete realities. Since, however, Darwinism has once for all displaced design from the minds of the 'scientific,' theism has lost that foothold...
William James, from Pragmatism