Webster tells us that teleology is "the doctrine of the final causes of things", but teleology is basically about meaning. If a belief is teleological, it means that it presupposes that everything within its sphere of interest has one overall purpose and is leading towards one unavoidable endpoint. Teleology is hence more a manner of thinking about things than it is a belief system in itself, and it is a manner of thinking about things that is so ingrained in modern thinking that we hardly even notice it is there anymore.
The father of teleology was Christianity. Polytheistic systems, in which there are multiple gods, tend to stress eternal conflict and balance, as in the Greek concept of hubris that I have already noded. Many ancient philosophers believed that history moved in constant cycles of birth and death, much like organisms. But with the emergence of Christianity, a whole new way of thinking about the world engulfed the West: Christianity said that the unfolding of human history was in fact the fulfillment of God's plan for mankind, and that this plan would eventually come to an end with the Apocalypse. Rather than history constantly zigzagging around, it moved in a linear direction towards an endpoint, which would be utopia.
One of Christianity's appeals is the certainty of this teleological system, which gives the believer a place in a preordained divine plan that has a meaning outside of himself; merely by living in certain ways, the believer has a place in a history that has been imbued with meaning. Any of the slings and arrows of outrageous fortune can be endured because they are interpreted as part of a divine plan, ineffable to man, which is by definition heading in the right direction. Lapses in faith come primarily from the suspension of belief in the existence of this plan, or about its inherent goodness; but so long as these two beliefs are maintained, it is a fine way of coping with the pain and disappointment of life (which, the cynic will be quick to note, approximates more accurately to what we are told to expect by non-teleological belief systems).
Teleology, however, has by no means vanished with the retreat of Christianity in the western world. When Enlightenment thinkers started their glasnost of Christian thought (those who believe these men aimed to destroy God are quite mistaken; they aimed to save Him, whatever the final result turned out to be) they did not abandon teleology. God's providence was instead replaced with the unfolding of human reason, which would run untrammelled through the world leaving "progress" in its wake. "Progress", a word that we hear used so frequently nowadays we have forgotten to think about what it really means, of course implies some sort of goal that is progressed towards. Progress towards what?
Marxism, one of the belief systems that replaced Christianity, is explicitly teleological. It even, when combined with Leninism, borrows some of the other trappings of Christianity. In Marxism, there will be a destructive, glorious war across the whole globe and the proletariat will inherit the Earth, sweeping away the unjust social order that preceded them. The exact details about the end-point were always hazy, and so was the path there; but we could rest assured history found its meaning in the unfolding of this path.
Liberalism - and note I mean liberal as in liberal, or what we usually now call democratic-capitalism or "western values" - also envisages the gradual unfolding of a free economic and political order across the planet, in which science and reason are utilized by man to create the perfect possible social order in which pain is minimized and pleasure maximized. The non-liberal world will eventually cease to exist and so hard choices do not need be made in compromising with it, and religion will eventually completely vanish as everyone signs on to the liberal journey towards "progress". The only difference between liberalism, Christianity, and Marxism is the exact nature of the utopian end-point, and the initial catastrophism of Marxism, which is why during the Cold War the world really wasn't big enough for both liberalism and Marxism, and there was always a tension between the two and Christianity.
All of this, along with the optimism about the utopian nature of the final outcome that Marxism shares with liberalism, shows the intricate and close links between Christianity and supposedly "rational" modern thought on human development, which presupposes a teleological end-point. Indeed, thought-systems such as Enlightenment rationalism and Marxism could hardly have triumphed over Christianity if they had not offered the belief in "progress" that displaced the Christian belief in God's providence; just like God's providence, liberalism and Marxism promise an end to war, poverty, disease and all else that ails man through the application of science. And they give any man who signs up to their project a definite and comforting place in history, with the gulag or the United States Air Force awaiting those who do not.
When the Cold War ended, liberal optimism finally encompassed the whole globe, or so we thought. Francis Fukuyama was the emissary of this teleology, saying that now Marxism was vanquished the whole world would naturally embrace liberal democracy, science and capitalism. And our generation had the opportunity to watch how foolish this was as cycles, chance and interruptions intervened in the historical process and set Russia and China off the course of western liberalism and down their own paths; Fukuyama's dream turned out to be just another narrative of human history with the west as its central protagonist, another trait inherited from Christianity.
For what history teaches us about teleology is that there is none. The belief in a supreme end-point - an ultimate cause for all actions that allows the world to be understood in a simple fashion - is always incorrect and leads to a dangerously naive view of the world. The belief that religion will inevitably succumb to reason, or that the Marxist version of reason will always succumb to the liberal version of reason, is to inject a purpose into history that simply does not exist. This can be seen most simply by observing that regression occurs and there is never absolute progress in ethical or political matters. World War I shattered the earlier version of liberal optimism in "progress" and the result was a return to irrationalism that spawned the Nazis, who had their own decidedly less pleasant version of teleology. As for our own optimism, all we are waiting for is to find out exactly how long we can maintain it before it too is shattered.