One of the most far-reaching consequences of the rationalism of the Enlightenment was the undermining of basic Christian faith among the educated classes. The effect was unintended because the project of many Enlightenment philosophers was to prove the existence of God using reason: Descartes and Leibniz assumed that God's existence could be rationally proved, indeed God was a necessary part of their philosophy.
There are many traditional "proofs" for the existence of God, and we will look at three of them: The argument from design, the ontological argument and the cosmological argument.
If you found a clock and examined the mechanism within it, you would probably think that this intricate mechanism was not the outcome of mere chance, that it had been designed.
Now look at the universe; is it possible that such an intricate mechanism, from the orbits of planets round the sun to the cells in your fingernails could all have happened by chance? Surely, this enormously complex mechanism has been designed, and the being that designed it must be God.
God is the perfect being. As He is most perfect, He must have all perfections. If God lacked existence He would not be perfect, as He is perfect he must exist.
Everything that exists has a cause. However, there must at some time have been a cause prior to all other causes. This 'prime mover' or first cause is necessary to explain existence. This first cause is God.
The French mathematician Blaise Pascal (1623-62) put forward an argument that would appeal to agnostics. (An agnostic is someone who believes that it is impossible to prove God's existence.)
His argument goes something like this: God either exists or he does not. If we believe in God and he exists, we will be rewarded with eternal bliss in heaven. If we believe in God and he does not exist then at worst all we have forgone is a few sinful pleasures.
If we do not believe in God and he does exist we may enjoy a few sinful pleasures, but we may face eternal damnation. If we do not believe in God and he does not exist then our sins will not be punished.
Would any rational gambler think that the experience of a few sinful pleasures is worth the risk of eternal damnation?
Kant attempted to show how philosophy could prove the existence of God. Unfortunately, for him his previous work showed that we could not know reality directly as thing-in-itself. What is real in itself is beyond our experience. Even if God exists, we can not know God as he really is.
For Kant the Christian could have faith in God, and this faith would be consonant with reason and the categorical imperative. Given that human beings have the autonomy to create moral values, it would not be irrational to believe in a God who gives purpose to the moral realm.
Hegel thought that the God of religion was an intuition of Absolute Spirit or Geist. Hegel's Geist is not like the transcendent (outside of our consciousness) God of traditional Christianity. For Hegel God is immanent and when we have understood that history is the process of Geist coming to know itself it appears that we are all part of Geist, or God.
For Feuerbach and Marx religion is seen as the projection of the human essence onto an ideal: God does not make man. Rather "God" is the invention of human consciousness. Marx also sees that religion is part of an ideological view that encourages the oppressed to accept their fate. As he says: "Religion is the sigh of the oppressed creature, the sentiment of a heartless world, and the soul of soulless conditions. It is the opium of the people.
"The abolition of religion as the illusory happiness of men, is a demand for their real happiness. The call to abandon their illusions about their condition is a call to abandon a condition which requires illusions."
Kierkegaard is regarded as the first existentialist.
"Have you not heard the madman who lit a lantern in the bright morning hours, ran to the market place and cried incessantly, 'I seek God!, I seek God!' ... Why, did he get lost? Said one. Did he lose his way like a child? Said another. Or is he hiding? Is he afraid of us? Has he gone on a voyage? Or emigrated?... The madman jumped into their midst and pierced them with his glances.
"'Whither is God'? He cried. 'I shall tell you. We have killed him - you and I. All of us are his murderers...'"
"...the madman fell silent and looked again at his listeners; and they too were silent and stared at him in astonishment. At last he threw his lantern on the ground, and it broke and went out. 'I came too early,' he said then; 'my time has not come yet. This tremendous event is still on its way, still wandering -it has not yet reached the ears of man."
In these passages Nietzsche is showing the inevitable unfolding anthropocentrism (lit. putting man at the centre of the world) implicit in philosophy since Kant. If we view our existence through human categories, then our concept of God is itself a human creation.
Nietzsche is not simply asserting his atheism; he is suggesting that once we are aware that the concept of God is our own creation we can no longer base our religious and moral beliefs on any notion of a divine external reality.
In the period that Nietzsche was writing, the death of God was just beginning. Western thought was starting to face the prospect of a radical change in its orientation, and it wasn't quite ready to own up to it yet.
Kierkegaard and Nietzsche represent opposite reactions to the inability of rationality to give a rock solid theoretical proof of God's existence. Kierkegaard calls for us to embrace God even if it seems an absurdity, while Nietzsche says it is time for us to create a new mode of being, with human creativity at its centre.
The atheist existentialist Sartre accepted God's death and much of his writing is attempt to look at the human condition in a world that is without a prime mover who could have provided a basis and structure for the understanding of being.
Anglo American analytic philosophers of the twentieth century have tended to agree that philosophy may help us clarify religious concepts, without giving us a secure foundation for religious belief.
Many people claim to have had a religious experience, to have experienced the divine directly. This experience is direct and is of a different quality to sensory experience or intellectual discovery, and therefore outside of the scope of philosophy.
The view that the existence of God cannot be proved or disproved by philosophy has not stopped developments in modern theology. Theologians are attempting to balance the anthropocentric view of God presented by philosophers since the Enlightenment with the need to provide a spiritual path and a guide to an ethical and meaningful way of life.