Science is defined both by its aims and its methods. It aims to discover the facts, the truth, about the natural and human worlds. It aims to be objective; that is, to seek the truth independently of our own wishes and desires, to remove personal values from the inquiry. In other words, it seeks to make positive statements of fact or logic and to eliminate normative, or value, statements: to say what was, is, or will be and not what ought to be.
A science is also defined by its methods. To be scientific is to be methodical, systematic, precise, orderly, logical, etc. Many scientists would accept the hypothetical-deductive model of the scientific method outlined by Karl Popper. First, we make assumptions or hypotheses. Second, we form deductions, predictions, implications and conclusions from these assumptions. Then we test these deductions by observation and experiment to discover evidence for them. If the evidence supports the theory, we accept it; if it does not quite fit, we modify the theory in the light of the evidence; if it does not fit at all, we reject the hypothesis and seek an alternative which better fits the evidence.
Science cannot be dogmatic. It is not infallible because scientific theories are subject to revision. Since we can never have a logical guarantee that a theory will not be falsified, we can never claim that we are in possession of the final truth. Thus the 'truths' of science are held 'until further notice'. This implies that science is open-minded. It also implies that a theory is not scientific if it could not conceivably be refuted by contrary evidence. Clearly, therefore, a crucial aspect of a truly scientific approach to any matter is the critical attitude, which works with the judgements confirmed by experience thus far, but holds even the best confirmed views in principle ready for modification or even complete replacement.
Religion is open to persuasive definition. It may be taken to mean simply a commitment to certain ideals of life - any committed faith. In this sense, Humanists could be described as religious. However, most Humanists dislike this description because in the Western World of Jewish-Christian traditions 'religion' means something more specific. It almost always refers to a creed involving belief in a personal god who created the universe, who commands our obedience and deserves our worship, and who made us immortal.
Religion in this sense does claim to be scientific because it offers explanations of the universe and our place in it. The monotheistic tradition maintains that there is one and only one god, that he is all-knowing, all-powerful and all-loving, that he created the universe, that he made human beings as special creatures with a soul, and that when we die this soul survives our body in 'another place'. The Christian tradition makes further claims. Until recently, all Christians believed that god intervened in this world in the form of Jesus Christ. However, many Christians would now reject this claim that Christ was god incarnate and argue instead that he was a prophet who embodied many of the best Christian values.
Many of the assumptions of religion in the sense outlined above have been refuted or at least challenged by science. Physics indicates that we are not living in the centre of the universe but in the backwoods of space. There may well have been a Big Bang but it does not follow that everything began in this way. It is entirely possible that an infinite universe contains local pockets of expansion and contraction. What we call 'the universe' may therefore be only one of many possible universes, or a region of a single universe, each of which has its own scientific laws. And none of these universes necessarily required a creator to get going. They may simply have arisen from random quantum fluctuations.
Biology indicates that we are part of nature and descended from other animals. "Slow, gradual, cumulative natural selection is the ultimate explanation of our existence" (Richard Dawkins: The Blind Watchmaker). If humans and other animals have a common ancestry, we cannot have been created with a soul at the beginning, unless they too have souls. All the biological evidence suggests that we are mortal, like all living creatures.
The social sciences also challenge religion in various ways. Anthropology views its origins in terms of fear of the unknown, sociology regards it as a means of social control or the worship of 'community', and Freudian psychology sees it as a projection of the need for a secure father-figure. The perspective of the social sciences leaves us with the possible conclusion that religion is not really a cognitive belief at all, based on reason and intellect, but an emotive belief, based on need.
The methods by which religion reaches its conclusions are also totally unscientific. Ultimately, it relies on faith - which is literally 'belief without reason'. We might as well have faith that the moon is made of green cheese. 'Faith' is not a word in the scientific dictionary