Christian theological assertions are illogical and highly improbable, but those faults have almost no place in a proper denial of those assertions. Religion is the irrational core of every worldview, of every belief system, mindset or way of looking at the world. It’s currently fashionable for so-called New Atheists to castigate mainly Christians and Muslims for the palpable irrationality of their religious beliefs, as though the issue that separates so-called secularists and theists were the Manichean conflict of Faith versus Reason. No non-autistic or otherwise sane atheist is a hyper-rationalist, a Data-like figure who turns solely to reason in all her affairs, never speculating, feeling, intuiting, trusting, or caving to higher powers. A viable defense of atheism doesn’t reduce to the following argument: (1) A worldview should be fully rational; (2) Theism is irrational; (3) Therefore our worldview shouldn’t be theistic. A person does not live by Reason alone. As the sociologist Emile Durkheim explained, you’re bound to form a religion around what you hold to be of ultimate importance. I’d add that only a machine truly cares about nothing, which implies that all people, all clever animals with primitive emotions and instincts are religious, although our religion needn't be theistic.
Indeed, those atheists who rest their case by showing that theists commit various fallacies and that their key assumptions are preposterous, reveal their irrational commitment to certain unexamined philosophical assumptions of their own, be they pragmatic, positivistic, or scientistic. The issue, then, isn’t whether a person should reject all religions as foolish, but rather which religions should be discarded. When you appreciate that logic and science stop short of fully justifying a worldview, that a human brain’s perspective on the world should be coherent, which means that a worldview should satisfy all of our cognitive faculties, including the rational and irrational parts of our mind, you should find yourself adopting subtler criteria in choosing what to believe at the philosophical or religious level. (For more along these lines, see Theism, Scientism, and Scientific and Philosophical Atheism.)
Now, Christianity happens to be execrable, but the pseudo-rationalist underestimates the religion’s inadequacies, by banally demonstrating that Christianity isn’t perfectly logical or scientific because, after all, the Bible contradicts itself and Jesus allegedly performed miracles. Proving as much shows only that Christianity fails as a mathematical proof or as a scientific theory, and such a demonstration would thereby in turn amount to a category error. Christianity contends for people’s religious commitment, and thus the religion’s inconsistencies and improbabilities are relatively insignificant.
The more loathsome aspects of the religion, to my mind, are ethical and aesthetic. What I mean is that the religion fails now, in modern and postmodern times, to uplift as a work of imagination; on the contrary, in the present context, Christian belief degrades a person’s character. When combined with modern myths and values--as every current, responsibly-held worldview must be--Christianity’s shortcomings are outrageous. The point, though, isn’t just that Christianity contradicts modern truths that should be taken for granted, which it obviously does, but that a synthesis of Christianity and modernism would make for an atrocious, wildly incoherent work of art that disappoints rather than fortifies. This is the Nietzschean point. What appalled Nietzsche wasn’t some assortment of petty cognitive defects of the religion, but the anachronism of Christian values, the anticlimax of the Christian narrative, the unethical effect of the religion which is to reconcile the gullible masses to secular excesses rather than energizing people with stories (myths) worth trusting.