EMPIRICISM: This theory denies the non-existence of not only God but all existents that are metaphysical, non-experiential and non-metaphysical. Proponents of this, school of thought are Francis Bacon, John Locke, Berkeley and it came to its highest point in David Hume. For these philosophers only what is experientially knowledgeable has got true knowledge. So for them only the empirical existence is real existence.
Berkeley formulated a doctrine based on this that to be is to be perceived. But do we sense God? So this Godâ€™s inability to be sensed makes him not existing.
SKEPTICISM: This shares the same origin of true knowledge only base on sense perception. Its basic tenet lies in doubt. It doubts everything that is. As in relation to empiricists who hold on seeing it even if skeptic sees it he still doubts it he even doubts his capability of knowing at all. David Hume doubted not only Godâ€™s existence but also doubted the possibility of knowing external reality at all. God for Hume is one of the ideas formed by habit of referring to a first cause as explanatory of all.
AGNOSTICISM: This holds that we cannot know any given reality e.g. God; whether he is or not. Hume, Kant, Voltaire and Bertrand Russell are agnostics. For them hence is no natural reason to establish the existence of a supreme being. The agnostic has no belief in God and has no reason to accept the arguments trying to rationally prove Godâ€™s existence. The agnostic only indirectly deny Godâ€™s existence and he suspends all attempt to try to provide such belief or proving Godâ€™s existence for it is beyond human mind to demonstrate that God is, as Kant say though we think it we cannot know it.
RATIONALISM: Immanuel Kant attacked with passion all traditional argument for Godâ€™s existence. He showed in this work that any consistent reasoning into Godâ€™s existence is fraught with erroneous syllogisms. The rate of disagreement amongst metaphysicians clearly shows that reason alone cannot demonstrate his existence or his no-existence.
POSITIVISM: This school of thought whose main proponent was August Compte. For him we know nothing except experience and its object of concern is humanity and not a transcendent and inaccessible God. This gave birth to logical positivism that denies both God and all metaphysical reality. The members of Vienna circle synthesized this theory: Carnap, Hampel etc. For this group of philosophers meaningful propositions are empirically verifiable, therefore metaphysical statements like God, and spirits etc are senseless because they are non-experiential. They are pure expressions of feelings.
SECULARISM: A new system of theoretical atheism was given birth to in the advent of the renaissance i.e. Period of Enlightenment. This system maintains that everything in the world can be accounted for by science, without recourse to God, even the God hypothesis can be accounted for by science. This in other words means secularism. In this way, the atheist opposes that the world or man can rely solely on the findings and proofs on science and put away God. All questions about reality are and can be answered without appealing or seeking reference to God. It combats every religion as a harmful aberration. It is indeed militant.