While an undergraduate, Flew attended the weekly meetings of C. S. Lewis's Socratic Club fairly regularly. Although he found Lewis to be "an eminently reasonable man" and "by far the most powerful of Christian apologists for the sixty or more years following his founding of that club," he was not persuaded by Lewis's argument from morality as found in Mere Christianity. Other philosophical proofs for God's existence also fail, according to Flew. The ontological argument in particular is false because it is based on the premise that the concept of Being can be derived from the concept of Goodness. Only the scientific forms of the teleological argument impress Flew as being decisive.
In God and Philosophy (1966) and The Presumption of Atheism (1984), Flew earned his fame by arguing that one should presuppose atheism until evidence of a God surfaces. He still stands behind this evidentialist approach, though he has been persuaded in recent years that such evidence in fact exists, and his current position appears to be deism. In a December 2004 interview, he said: I'm thinking of a God very different from the God of the Christian and far and away from the God of Islam, because both are depicted as omnipotent Oriental despots, cosmic Saddam Husseins.
On several occasions, apparently starting in 2001, rumours circulated claiming that Flew had converted from atheism. Flew refuted these rumours on the Secular Web website. In 2003, he signed the Humanist Manifesto III.
In December 2004, an interview with Flew conducted by Flew's friend and philosophical adversary Gary Habermas was published in Biola University's Philosophia Christi, with the title Atheist Becomes Theist - Exclusive Interview with Former Atheist Antony Flew. Flew agreed to this title. According to the introduction, Flew informed Habermas in January 2004 that he had become a deist , and the interview took place shortly thereafter. Then the text was amended by both participants over the following months prior to publication. In the article Flew states that he has left his long-standing espousal of atheism by endorsing a deism of the sort that Thomas Jefferson advocated ("While reason, mainly in the form of arguments to design, assures us that there is a God, there is no room either for any supernatural revelation of that God or for any transactions between that God and individual human beings."). Flew states that certain philosophical and scientific considerations had caused him to rethink his lifelong support of atheism. However, it is clear from the interview that Flew is not comfortable with either Christianity or Islam.
Flew's conception of God as explained in the interview is limited to the idea of God as a first cause, and he rejects the ideas of an afterlife, of God as the source of good (he explicitly states that God has created "a lot of" evil), and of the resurrection of Jesus as an historical fact. He is particularly hostile to Islam, and says it is "best described in a Marxian way as the uniting and justifying ideology of Arab imperialism."
Flew has subsequently made contradictory statements to those given in the Habermas interview as justification for his endorsing of deism. In October 2004 (before the December publication of the Flew-Habermas interview), a letter written to Richard Carrier of the Secular Web, stated that he was a deist and also said that "I think we need here a fundamental distinction between the God of Aristotle or Spinoza and the Gods of the Christian and the Islamic Revelations.". Flew also said: My one and only piece of relevant evidence [for an Aristotelian God] is the apparent impossibility of providing a naturalistic theory of the origin from DNA of the first reproducing species ... [In fact] the only reason which I have for beginning to think of believing in a First Cause god is the impossibility of providing a naturalistic account of the origin of the first reproducing organisms.
In an another letter to Carrier of 29 December 2004 Flew went on to retract his statement "a deity or a 'super-intelligence' [is] the only good explanation for the origin of life and the complexity of nature." "I now realize that I have made a fool of myself by believing that there were no presentable theories of the development of inanimate matter up to the first living creature capable of reproduction." wrote Flew. He blames his error on being "misled" by Richard Dawkins, claiming Dawkins "has never been reported as referring to any promising work on the production of a theory of the development of living matter". (Dawkins has - in "Evolutionary Chemistry: Life in a Test Tube," published in the 21 May 1992 issue of Nature, with Laurence Hurst.) The work of physicist Gerald Schroeder had been influential in Flew's new belief, but Flew admitted to Carrier that he had not read any of the scientific critiques of Schroeder that Carrier referred him to.
When asked in December 2004 by Duncan Crary of Humanist Network News if he still stood by the argument presented in The Presumption of Atheism, Flew replied he did but he also restated his position as deist: "I'm quite happy to believe in an inoffensive inactive god". When asked by Crary whether or not he has kept up with the most recent science and theology, he responded with "Certainly not", stating that there is simply too much to keep up with. Flew also denied that there was any truth to the rumours of 2001 and 2003 that he had abandoned his atheism or converted to Christianity.
A letter on Darwinism and Theology which Flew published in the August/September 2004 issue of Philosophy Now magazine left the world hanging when it closed with, "Anyone who should happen to want to know what I myself now believe will have to wait until the publication, promised for early 2005, by Prometheus of Amherst, NY of the final edition of my God and Philosophy with a new introduction of it as ‘an historical relic’."
But in 2005, when God and Philosophy was republished by Prometheus Books, the new introduction failed to conclusively answer the question of Flew's beliefs. The preface says the publisher and Flew went through a total of four versions (each extensively peer-reviewed) before coming up with one that satisfied them both. The result is an introduction, written in a distinctly detached third-person context, which raises ten matters that came about since the original 1966 edition. Flew refrains from personally commenting on these issues, and basically says that any book to follow God and Philosophy will have to take into account these ideas when considering the philosophical case for the existence of God.