The American philosopher, Michael Novak began his 1982 lecture in Sydney with the observation:
Not many would have predicted as World War II drew to its bloody close, that the two most powerful currents in world affairs at the end of the twentieth century would be religion and ethnicity. Most observers then thought that the world then would become inexorably more homogenized, more rational, more scientific.
This observation is valid today even more than in 1982. In the West ethnicity (nationality) as a positive cultural trait is in danger of degenerating into irrational populism, andreligion, notably Christianity but now also Islam, is in danger of getting lost between the extremes of fanaticism and irrelevance. Ethnicity and the links between ethnicity and religion are not my concern here. My concern is religion as such, and even then only its "metaphysical" dimensionor level, bracketing its other, ethical/moral, aesthetic/emotional, ceremonial/ritual, etc,dimensions. On this level religious believers encounter others, including those who do not subscribe to any religion but have ideas about what is reality. In other words, my concern here is the existence or not of God.
A theist believes in (the existence of) God, an atheist does not. This simple definition assumed God as a self-explanatory concept within the tenets of Christianity with no distinction between its noumenal essence (Kant's thing-on-itself) and phenomenal attributes (as perceived). The presence of Islam as an earnest competitor to Christianity in the West, but mainly the advance of science, leading to a more sophisticated philosophy of science, notably physics, call for a deeper look at the theist-atheist encounter.
This encounter can be inspired by insights from philosophy of science as was medieval philosophy and theology inspired by insights from Greek philosophers likePlato and Aristoteles. Paul Davies, in his God and the New Physics, goes even so far as to claim that "it may seem bizarre, but in my opinion science offers a surer path to God than religion". One does not have to go that far, but Ian G Barbour's ground breaking Myths, Models, Paradigms: The Nature of Scientific and Religious Language (1974), and research built on it, offers many inspirations in that direction. For instance, if some properties assigned to God seem to go against common sense (triune God, the "problem of evil"), so do some properties assigned to matter (the enigmas of quantum mechanics). Put simply, in both physics and metaphysics, one needs to go beyond a naive common sense approach.
My default position here is that of what is called methodological atheism: there is no need for God to explain the working of the physical world. The theist perspective appears only for those who ask questions about this world that cannot be answered, or made sense of, from within science, like the famous (metaphysical) question "why there is something rather than nothing" posed by Gottfried W. Leibniz and other philosophers.
To be precise, I shall split the basic belief in God into two parts:
Pre-theist world view assumption (belief): Reality - objective, all that exists independent of whether or not thought of by humans - has aspects or features that are principally not accessible by (natural) science; these aspects one calls transcendent, (divine, spiritual, supernatural). Aspects that are accessible through scientific investigation are called immanent and phenomena with solely immanent aspects constitute the physical world. God, if assumed, is transcendent or at least has transcendent aspects/features.
Thus a pre-theist assumes the existence of a transcendent reality that is neither pure thoughts (figments of imagination) nor accessible through senses, aided by instruments or scientific theories. Physical world or reality is then all that is accessible through senses, aided by instruments or scientific theories.
I am not going to give a philosophical definition of God, only mention that Aquinas sees God not as ens summum (the highest being) but as ipsum esse subsistens (the sheer act to be itself), resonating with Paul Tillich's contention that God is not merely "a being" but "being itself", these distinctions not very comprehensible to a modern, natural science oriented, mind.
In this sense not only (religious) Jews, Christians, Muslims and adherents to some other religions, but also pantheists, panentheists, deists, etc. satisfy the pre-theist assumption, since they all do not think God as such can be investigated by scientific methods. The same about God as understood by Einstein or Spinoza. They might not have liked the concept of transcendence, so I shall leave it open whether they could be regarded as pre-theists, although according to Karl Jaspers Spinoza, believed that "God's transcendence was attested by his infinitely many attributes, and that two attributes known by humans, namely Thought and Extension, signified God's immanence". Similarly the recent convert Anthony Flew. Buddhists, at least some, probably also satisfy the pre-theist assumption, where the preferred concept is that of Nirvana rather than transcendence or God.
Theist view of God (belief): The pre-theist belief in transcendent God is concretised into a belief in a personal God (eventually within a wider spiritual world) ie God is seen by theist religions - Judaism, Christianity, and Islam - as the "Creator" (i.e, its cause and purpose) of the world" who can and will communicate with mankind (Revelation, divine action, prayers). To borrow from philosophy of science each theist religion models (represents) God as a person with these or those additional attributes, different for different religions. Like in physics, where the concepts of eg space and time are modelled (mathematically) differently in Newtonian physics, special relativity, general relativity or quantum mechanics, providing different explanations. In physics there is a way of deciding (through measurement and observation) which model, which explanation is preferable in a given context, whereas in the case of God there are arguments (not proofs or evidence) for this or that particular "model of God and the transcendence", but the final arbiter is personal faith.