Objective morality makes atheism improbable

In arguing for the contention I want to defend two basic props: Firstly, (1) Objective morality exists; and secondly, (2) If atheism is true, then objective morality does not exist. If there is reason for these two, then there is reason for (3) Atheism is improbable Reasons for (1) To reiterate, I understand objective morality to include facts about what people should do, and facts about what is right/wrong, good/bad, which are independent of what people believe them to be. As such, I think there many good reasons to be made for considering (1) to be true. Reason A: universal judgments presuppose objective morality By 'universal judgment' I mean a judgment about what is good or bad, that holds for more than the speaker and his community. Universal judgments are made when we claim that moral progress occurs, e.g. when we claim that the abolition of slave trade is a good thing simpliciter. Given that such a judgment is prima facie true, there is prima facie reason to think that objective morality is true. Reason B: indefeasible duties presuppose objective morality By an indefeasible duty I mean a duty that can't be excused in any circumstances absolutely. It would be always and everywhere absolutely wrong to gratuitously harm another person, for instance (e.g. torturing a child for fun). Rejecting that such indefeasible duties hold, amounts to allowing for the possibility that torturing a child for fun is permissible in some circumstances (because the duty not to is defeasible). This is not right. Thus, there is prima facie reason to think that objective morality is true. Reason C: Rational discourse presupposes some objective moral norms In order for there to be a rational conversation, there is the presupposition that there is an objective norm about how we should think—namely, that accepting truthful rational argumentation is desirable, and vice versa. I.e., it is good to think in a certain way (truthfully and rationally) and bad to think another way (untruthfully or irrationally). It is impossible to engage in rational discourse without this presupposition because the rejection of an argument presupposes the objective duty not to be convinced by bad arguments! The rejection of such a norm amounts to the rejection of the right to consider an irrational person with false beliefs having a duty to think otherwise. But this is undesirable. Again, another prima facie reason to think that objective morality is true. Reasons for (2) If atheism is true, I presume that there is no transcendent cause of the universe (intentional or otherwise), and hence, there is no underlying purpose to physical reality. In light of this, reality seems (in theory) capable of complete description via natural scientific methods. That is to say—reality can be reduced to descriptive facts. However, objective morality is fundamentally normative. In light of this, what objective fact about the world corresponds to objective moral facts? Evolution? Biological human desires? The community in which we live? The legal system? Our interior conscience? I take it that it that there is prima facie reason to believe that norms cannot be derived from facts (Cf. Moore, Hume), and that hence, there is prima facie reason for atheism being unable to account for the existence of moral facts. To my mind, the most plausible account of objective morality given atheism is error theory—i.e. Recognising that moral facts appear to exist, (because so many parts of human discourse and behaviour presuppose it), but considering them to be false, due to their not corresponding to any objective reality. However, error theory is likewise highly implausible, because it requires a drastic revision of our fundamental human intuitions about our moral discourse. (I.e. I assume here that there is good reason to think that fundamental human intuitions are analogous to sense-perception; the burden of proof is on the skeptic to show that there is sufficient reason to reject such intuitions and perceptions as being truthful/veridical). - Having given reasons for (1) and (2), I take it that there is reason to believe (3).