The foundation of Metaxas's argument is the striking correlation between the physical conditions of the Earth and the needs of organisms that live on that Earth. As he puts it:
Today there are more than 200 known parameters necessary for a planet to support life—every single one of which must be perfectly met, or the whole thing falls apart.This correlation is something any knowledgeable observer should accept; the match between organisms and their environments is quite striking. Given that correlation, we naturally want to understand the cause, and that is where Metaxas's views differ so radically from those of most scientists.
There are really two possibilities. The first explanation argues that life is picky, fragile, and static, and that the universe must have been adjusted to the particular needs of living things. This is where Metaxas's description, that "every single [parameter] must be perfectly met" comes from. If the universe needed adjusting (often called "fine-tuning" when describing the constants of physics), then there must have been an intelligent being doing the adjusting - hence "the Case for God."
The other possibility is that the conditions of the universe are relatively static and not "adjusted" to benefit life, but that living things are capable of evolving so that they can tolerate those conditions (at least in a few places like Earth). Either explanation could produce the striking correlation; how do we choose between them?
Each explanation makes strong demands on our understanding. One particularly strong demand for the fine-tuning explanation, often unstated or ignored, is a complete understanding of the possible ways living things could exist. Arguing that the universe has to be adjusted to the requirements of Earth life includes the implicit assumption that the Earth includes the complete sweep of potential living things. If you want to argue that Metaxas's "200 parameters" have to be the way they are, you have to argue that the only way living things could possibly exist is pretty much just the way they do here on Earth.
As an example, if you insist that life absolutely requires liquid water, having planets with liquid water on their surface seems like a reasonable demand of the universe. Can you be sure of that assumption when the only examples of living things you know of all come from a planet with oceans? It's rather like insisting that all mammals deliver well-developed live young, completely ignorant of the marsupial branch of the mammal family tree. Time has not been kind to this assumption of "comprehensive imagination." That living things could survive in deep oceanic hydrothermal vents (way too deep to get energy from the sun), at the near-boiling temperatures of hot springs, or in extremely acidic conditions were all hardly conceived (not even taken seriously enough to be considered impossible) until the organisms living in those environments were discovered. Claiming to imagine every possible way of being alive seems both remarkably arrogant and extremely unlikely. To borrow from Daniel Kahneman's remarkable book, this explanation suffers from the "WYSIATI" fallacy: What You See Is All There Is.
But what about the other explanation - that life has evolved to match the conditions in which it finds itself? That lineages of organisms can change over time was once considered an extraordinary idea, and one might rightfully demand some pretty strong evidence of its reality. After over 150 years with the idea of natural selection, the evolutionary biologist can point to a range of different types of evidence, from change in fossil lineages, to the hierarchical pattern of diversity of living things, to the underlying biochemical similarity of all living things. One can observe the evolutionary process in the lab with organisms whose generation times are conveniently short, as well as in decades-long studies of wild populations. We even know that life on Earth has adjusted to huge shifts in the composition of its atmosphere caused by living things themselves! In short, that lineages of living things on Earth can change over time is a well-documented fact. That other forms of life could evolve does not seem far-fetched, since there is no apparent reason the conditions for evolution (reproduction, a genetic system, and differential survival) could not be fulfilled in radically different forms of life.
So in understanding the strong correlation between environment and living things, we can either claim that life is inherently dependent on the conditions here on Earth (again, a claim of comprehensive imagination for which we cannot have any real basis) or claim that life can adapt to at least some of the environments available (for which we have abundant evidence, at least for Earth-life). If we are really honest about the limitations of our examples of life - and the limits of our imaginations - we should see the absurdity of the assumptions necessary to claim proof of supernatural fine-tuning. The far more plausible explanation is the evolutionary one, and it is the only one with sound empirical evidence.
(note: not long after I wrote this post, the AAAS hosted a session on what a "shadow biosphere" (i.e. not based on DNA/protein) might be like. There's an account of the session here, if you're curious.)