The "presumption of Atheism", per Wikipedia, was not accepted by Flew's peers for 3 decades:
Flew's proposal to change his profession's use of the term atheism saw limited acceptance in the 20th century, but in the early 21st century Flew's negative sense of 'atheism' came to be forwarded more commonly. The impact of Flew's proposed negative atheism, which is often referred to today as 'weak atheism' or 'soft atheism', is illustrated by analytic Philosopher William Lane Craig's 2007 assessment that the presumption of atheism had become "one of the most commonly proffered justifications of atheism." And BBC journalist William Crawley 2010 analysis: "The Presumption of Atheism (1976) made the case, now followed by today's new atheism, that atheism should be the ... default position". In recent debates, atheists often forward the Presumption of Atheism referring to atheism as the "default position" or has "no burden of proof" or asserting that the burden of proof rests solely on the theist.
If one accepts the principle of sufficient reason, then every position, either for or against accepting a thesis, needs to be justified. This is illustrated in the following diagram:
Per the PSR, the "presumption of atheism" is a fallacy of shifting the burden of proof.
Flew himself argued strong agnosticism -- that Theism is incoherent, and this was his justification for atheism. Upon the publication of Swinburne's "The Coherence of Theism", Flew admitted his claim to incoherence was untrue, and this opened him to considering theism. By the end of his life, he had adopted deism. The publication of ghost-written books under his name, which trended toward theism rather than deism, are suspect, given his advancing age and declining faculties.