Interestingly, my favorite atheistic philosopher, Jean-Paul Sartre, was rather opposed to the concepts of the modernists. Though most would agree that he was quite consistently rational in his conclusion that a world without God/meaning is pure misery, Sartre insisted that Reason is not the characteristic that sets mankind apart. Instead, he insisted that the Will is the primary quality of human beings (not that he affirmed any human nature).
This aspect of existentialism (Sartre's philosophy) was not unique to atheists. Soren Kierkegaard (of whom I've written before) believed that it is the Will that motivates faith. In fact, according to Kierkegaard faith is directly contrary to reason (as in the case of Abraham's near-sacrifice of Isaac). It is only the Will, the actual choice to live and act for God rather than for the praise of men, that distinguishes the Christian from all others.
Fascinating as this all is, it didn't come to life in my mind until I was reading Lilith and a few of George MacDonald's Unspoken Sermons.
Troubled soul, thou art not bound to feel, but thou art bound to arise. God loves thee whether thou feelest or not. Thou canst not love when thou wilt, but though art bound to fight the hatred in thee to the last...Will thou his will...Heed not thy feelings: do thy work. As God lives by his own will, and we live in him, so has he given to us power to will in ourselves. How much better should we fare if, finding that we are standing with our heads bowed away from the good, finding that we have too feeble inclination to seek the source of our life, we should yet will upwards toward God, rousing that essence of life in us, which he has given us from his own heart, to call again upon him who is our Life, who can fill the emptiest heart, rouse the deadest conscience, quicken the dullest feeling, and strengthen the feeblest will! (The Eloi)
Not to say (as Kierkegaard did) that there is no place for reason, or even for feeling, in the life of the Christian. Indeed, does not God call us to love Him with all our heart (feeling), mind (thinking), and strength (will)? But when reason or feeling fail, what can motivate us to do the good that God requires of us?
It is a choice to love God in the hard times, in the darkness, in the confusion, in the moments of uncertainty. Reason tells you it's absurd. Feelings aren't up to the task. But God promises to conform our Will to His own and in Him we live and move and have our being. "Existence was mine in virtue of a Will that dwelt in mine." (Lilith)