Where by "failure" I mean its gradual displacement from the center of the moral and intellectual life of they civilization.
To define these options bit:
Poll option one is the conservative answer. It holds that Christian belief would be as predominant today in the West as it was in 1913 if it were not for the conscious, deliberate machinations of a small group of secularizing elites promoting atheism and amorality.
My thoughts: This is the least tenable of the four options I've provided, in part because 'the elite' in the West has never been anti-Christian. To be sure, they are opposed to fundamentalism, but only because it is at odds with liberal-capitalist notions of 'progress'. The invocation of the defense of Occidental Christianity during the Cold War is proof-positive that Western elites want generally to employ Christianity to their own ends.
Poll option two is the liberal answer, the "secularization thesis". According to this theory, Christianity is doomed to deplacement, as are all religions eventually, by the gradual and wholly unconscious forces of mental and mechanical progress.
My thoughts: This is almost as problematic a solution to the question posed as the first answer. It assumes a great deal of the structure of Christian ideology - progress towards a "new Heaven and a new Earth", an eventual end to history, and so on - while draining it of its metaphysical content.
Option three is what I call the Nietzscheite option: Christianity has failed because it is inherently flawed. It can exist only among theoppressed, and as soon as a people become strong enough to shirk ofc a collective sense of inferiority it will abolish the correspondent notimon of individual existential guilt that informs Christianity.
My opinion: This is the view I hold closest to. Christianity, in a very real sense, requires weakness to thrive (it is little wonder that Christianity is ascendant today only in the impoverished Third World nations of sub-Saharan Africa, Latin America, and the American South). A strong people wants a religion of strength and severity.
Option four: The Marxist solution. Christianity belongs at the historical latest to the age of feudalism; the rising capitalists of the eighteenth and nineteenth centuries sought initially to do away with it altogether, as a reminder of the hated age of the nobility, and retain it only as a matter of practicalg politica expedience.
My opinion: This is superficially similar to the liberal answer, relying on notions of deterministic 'progress', but avoids some of its problems by acknowledging the fact of necessity and human action in historical processes, rather than ascribing all history to forces largely independent of men.