Secular morality is superior to religious morality

I will use an analogy to illustrate the flaws in my opponent's argument. Let's say there's a debate titled "Evidence-based medicine is superior to anecdote-based medicine." We would hear that since neither can be shown to be a perfect system, then it's equally valid to favor either one. A similar situation arises when people argue that since no one can be absolutely certain of either evolution or creationism, then both worldviews are equally valid, or "equally wrong." That's certainly not the case.Regarding my assertion that The Bible is wrong about slavery, my opponent posits that I would have to start with a presuposition about the validity of secular morality in order to make the assertion in the first place. Perhaps that's true, and I think it's illustrative to consider why my assertion would appear correct nonetheless to most rational observers. I'll get into that shortly, but let's also consider an equivalent assertion in the context of my proposed analogy. Suppose I said "homeopathy is wrong about its cancer treatment methods." The way to demonstrate this would be to test the efficacy of the methods, and how do you do that? With the methods of evidence-based medicine.A system of secular morality is the de facto system we live by, even if that's unacknowledged. The reason we believe slavery is wrong is because we've applied reason and debated the issue, with the benefit of substantial historical knowledge of what slavery is. No religious book has told us that slavery is wrong, nor did we need to be told. We are, in fact, capable of arriving at such moral understandings on our own. Even religious people apply and make use of secular morality to one degree or another. Religious precepts do nothing more than add a measure of confusion that is not based on reason or evidence.Religious morality does not have a solid foundationI do not accept that there's necessarily an transcendent absolute morality, which my opponent calls "actual morality", or a third system by which we can judge religious vs. secular morality. There's no evidence of this absolute/divine morality, so we can ignore it until such time there is any evidence of it. Morality is something we need to define in a way that is beneficial, much like we define "health." Who cares if there's an absolute/divine conception of "health"?Even if we were to accept that there is an absolute morality, we would still be left with the question of how we would find out what the precepts of this absolute morality are. There are various ancient texts that claim to make pronouncements about absolute morality, but there's no evidence that these books come from an absolute moral law giver.In other words, you end up with becase-The-Bible-said-so type claims. These claims are obviously not the end of the argument, because you can still ask for evidence that The Bible or The Koran are the word of God, and so on. In a secular moral system, you end up with much simpler premises that are more evident and easier to agree on.Epistemology and ontologyMy opponent's strategy is to argue that you can't judge which system is better, except by looking at what each system explains. A secular system does not make presupositions about the existence of a trascedental morality or a person who dictates said morality. A religious system does, and this supposedly makes religious morality a better explanatory model of reality, as it tells us where morality allegedly comes from. In other words, we're expected to accept that God-dit-it is an adequate way to resolve issues we're not certain about.Proposing the existence of entities we don't know exist actually makes an explanation less likely than one that doesn't require such entities.