… the more purely secular liberalism has become, the more it has spent down its Christian inheritance—the more its ideals seem to hang from what Christopher Hitchens’ Calvinist sparring partner Douglas Wilson  has called intellectual “skyhooks,” suspended halfway between our earth and the heaven on which many liberals have long since given up. Say what you will about the prosperity gospel and the cult of the God Within and the other theologies I criticize in Bad Religion, but at least they have a metaphysically coherent picture of the universe to justify their claims. Whereas much of today’s liberalism expects me to respect its moral fervor even as it denies the revelation that once justified that fervor in the first place. It insists that it is a purely secular and scientific enterprise even as it grounds its politics in metaphysical claims. (You will not find the principle of absolute human equality in evolutionary theory, or universal human rights anywhere in physics.) It complains that Christian teachings on homosexuality do violence to gay people’s equal dignity—but if the world is just matter in motion, whence comes this dignity? What justifies and sustains it? Why should I grant it such intense, almost supernatural respect?
If you click on Ross’s original above, you’ll see a link to Julian Sanchez’s response accusing Ross of making a circular argument. Ross explains why this is wrong, and adds:
I don’t think that secular liberalism’s dependence on Christian premises provides anything like a slam-dunk argument for why Christianity is true. But I do think that because secular liberalism has trouble given a coherent account of itself absent those premises, the confidence with which many contemporary liberals presume to stand in judgment over Christianity (on sexual ethics, especially, but much else as well) is unwarranted and even self-deluded.
Not incidentally, the British political philosopher John Gray, who is a secular nonbeliever, has written extensively about how Enlightenment-based progressivism depends heavily on the Christianity that it reacts against. I don’t have time at the moment to go through my Gray books to pluck out a representative passage, but this, from his book “Straw Dogs,” gives you an idea of his thinking:
Formerly philosophers sought peace of mind while pretending to seek the truth. Perhaps we should set ourselves a different aim: to discover which illusions we can give up, and which we will never shake off. We will still be seekers after truth, more so than in the past; but we will renounce the hope of a life without illusion. Henceforth our aim will be to identify our invincible illusions. Which untruths might we be rid of, and which can we not do without? – that is the question, that is the experiment.
I think it’s fair to say that Gray would accuse secular liberals of deluding themselves by concealing (from themselves) the extent to which the things they hold most dear can only be understood on the basis of a Christian metaphysic.
UPDATE: Noah Millman got into this discussion yesterday.  My answer to him would be that you don’t have to believe that Christianity is true to believe in human rights and dignity, but to say that God — by which I mean the basis of all reality — has ordained the universe such that x, y, and z are true is a much stronger basis than saying “these things are true because I believe them to be true.” For someone outside a faith or metaphysical belief system, it is plain as day that the true believers are only expressing their own preferences when they say “Jesus Christ was God” or “there is only one God and Muhammad is His prophet.” To a believing Christian or Muslim, though, this is untrue, and indeed the unbeliever does not recognize the Truth.
The point is that whenever one grounds one’s moral claims in religious revelation, they will be less persuasive (by definition) to unbelievers, but will have a stronger hold on the allegiance of believers. To tell a man that he must stop beating his wife because it violates her human dignity may be in some sense true, but to tell him that he must stop beating his wife because Almighty God will hold him responsible for his cruelty, and send him to hell, is likely to be a lot more persuasive to him. Unless, of course, he is an unbeliever, in which case … you see?