If there are countless universes with different physical constants, then a life-permitting universe like ours was bound to exist somewhere by chance, which removes the need to invoke a designer to explain fine-tuning.

Why people find it persuasive

Given enough universes with randomly varying constants, one that happens to permit stars, chemistry, and life is statistically expected rather than miraculous, similar to how someone winning a lottery is unsurprising if enough tickets are sold across enough drawings. Physicists like Max Tegmark and theories like eternal inflation and string theory's landscape of possible vacuum states give this idea real scientific grounding, not just speculation invented to dodge theism.

Important distinctions

• • Multiverse theories with independent empirical support versus multiverse invoked specifically to explain away fine-tuning
• • Anthropic selection effects versus an actual causal explanation of why a life-generating mechanism exists at all
• • Different multiverse proposals (eternal inflation, string landscape, many-worlds) with very different evidential status
• • Explaining why we observe fine-tuning given our existence versus explaining why any life-permitting universe exists at all
• • Whether the meta-laws generating universes in a multiverse are themselves in need of explanation, including possible fine-tuning

Christian response

• • The lottery analogy has real force for addressing observer selection: if a multiverse generating varied constants exists, it is unsurprising that observers find themselves in one of the rare life-permitting universes, since observers cannot exist to observe the others. This is a legitimate application of the anthropic principle and is not, by itself, a desperate dodge; serious physicists take multiverse theories seriously for independent reasons connected to inflationary cosmology and string theory, not merely to answer theism.
• • The difficulty is that no current multiverse proposal has direct empirical confirmation; eternal inflation and the string theory landscape are extrapolations from theories that are themselves incomplete and empirically underdetermined, and physicist and philosopher of science Paul Davies, among others, has noted that invoking an unobserved and unobservable multiverse to explain fine-tuning risks trading one unverified metaphysical commitment for another, since multiverses are exactly as untestable by current or foreseeable means as many theistic explanations critics of religion reject on those very grounds.
• • A deeper problem, pressed by philosopher Robin Collins, is that the mechanisms proposed to generate a multiverse (the inflaton field's properties, string theory's landscape structure) themselves require very specific underlying physical laws and parameters to produce universe-generating variation at all; this pushes the fine-tuning question back a level rather than eliminating it; a universe-generating mechanism capable of producing life-permitting universes is itself a nontrivial, specifically structured thing requiring explanation, not an explanatory free lunch.
• • There is also a question of whether the multiverse move over-explains: if literally any improbable outcome can be explained by positing enough unobserved trials, this reasoning pattern threatens to explain away any evidence whatsoever, since virtually any specific state of affairs becomes 'unsurprising' given infinite unobserved variation, which some philosophers, including Richard Swinburne, argue should make us suspicious of it as a genuine explanation rather than an unfalsifiable escape hatch.
• • None of this proves the multiverse is false; it may well exist, and many respected physicists hold it for reasons unconnected to the God debate. But as a response specifically to fine-tuning, it trades a directly evidenced puzzle (measured physical constants falling in narrow life-permitting ranges) for an unobserved, currently untestable entity whose own generative mechanism raises comparable explanatory questions.

Christian counterresponse

The relevant asymmetry is that theism is typically defended as the terminus of explanation, a necessary, non-contingent being whose existence is argued philosophically (through arguments like the kalam or contingency arguments) rather than an entity whose existence is inferred solely to explain away one specific piece of data; a multiverse, by contrast, is generally proposed as a further contingent physical structure requiring its own explanation, which is a different logical role even though both are currently unobservable.