Verificationists typically say that for a claim to be meaningful it must be empirically testable. Tossing a coin might test claims about gravity, mechanics, or the symmetry of the coin, but it does not test an unrelated claim. It is probably meaningful to believe that Christ will return in 10,000 years (so long as we're specific about what "Christ" and "return" mean) but that does not mean it is plausible. In thinking about what is rational to believe we need to consider both meaningfulness and plausibility.

There is more to being a Christian (or a Jew, or a Muslim, or Buddhist) than subscribing to a particular set of beliefs. Religions incorporate ritual, culture and history as well as beliefs. It is a matter of opinion whether or not beliefs are "fundamental" or "necessary" for membership in a religion. (Some Christian denominations require it, but not Christianity as a whole, and not most branches of other religions). Indeed, I self identify as an atheist and a Jew and see no contradiction in doing so. Most atheists have other identifications (religious, political, ethical, national etc) since identifying yourself in terms of what you don't believe isn't really much of identity.

Actually the universe is not infinite, although it is extremely large. Your claim that "it is inevitable that at least one of the millions of planets has the required characteristic for life" is not a logical claim (it is not true because it follows the laws of logic); it is an empirical claim and depends crucially on the likelihood of (a) a suitable planet and (b) suitable evolution on that planet. We don't yet know how likely or unlikely the evolution of life is, because our understanding of evolution, and life, is paltry. Intelligent design theorists take advantage of this ignorance to argue that a designer was necessary for the wonderful yet highly improbable complexity of life. But in fact we do not know how (un)likely life is in this universe or how a designer might work in the universe.

Actually the universe is not infinite, although it is extremely large. Your claim that "it is inevitable that at least one of the millions of planets has the required characteristic for life" is not a logical claim (it is not true because it follows the laws of logic); it is an empirical claim and depends crucially on the likelihood of (a) a suitable planet and (b) suitable evolution on that planet. We don't yet know how likely or unlikely the evolution of life is, because our understanding of evolution, and life, is paltry. Intelligent design theorists take advantage of this ignorance to argue that a designer was necessary for the wonderful yet highly improbable complexity of life. But in fact we do not know how (un)likely life is in this universe or how a designer might work in the universe.

I think you are right to discern that the "playing God" objection to stem cell research/cloning is not what it seems to be. Those who offer this objection seem quite comfortable with the idea of "playing God" in well proven medical interventions e.g. appendectomy for appendicitis, C-section for obstructed labor, chemotherapy for leukemia. Of course there are some people (Christian Scientists, for example) who forgo most medical care on religious grounds. But the vast majority of those who worry about "playing God" with stem cell research/cloning are happy consumers of the best that health care has to offer. The question is, why don't they extend that happy consumer attitude to stem cell research/cloning? I am not sure of the answer to this, but I think it may have to do with the uncertainty that currently exists around these new technologies (will they work? will they produce monsters of some kind?). So it may be a risk aversive attitude of the kind "leave it to God, we don't know enough to...