Before the contrast, let me state the assumption I'm working with, an assumption people might disagree with.
My assumption is that everyone, theist and humanist, is involved in metaphysics. Specifically, systems of ethics and/or life philosophies (visions of a "good life") are inherently evaluative and axiomatic. That is, some values have to be taken as non-negotiable, axiomatic givens. For example, the inviolable dignity of human life. A historical example is the Declaration of Independence: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights..."
These self-evident, axiomatic values are metaphysical in that are not and cannot be derived from the laws governing the material cosmos. If anything, a rigorously empirical and reductionistic investigation of the cosmos undermines and sits in tension with axiomatic values. There are no values to be found the the equations of particle physics. Nor meaning in the Periodic Table. The more you drill down into the "building blocks" of nature the more devoid of meaning and value the world appears to be.
So that's my starting assumption. Everyone, theists and humanists, are engaged in some axiomatic evaluation of the world. Some values are taken to be non-negotiable givens.
Now if that's true, as I believe it is, it allows me to make the following contrast between a religious and a humanistic metaphysics, and why I think a religious metaphysics is better than a humanistic metaphysics.
Specifically, a religious metaphysics takes the self-evident, axiomatic, non-negotiable goods as real. Religion argues for values flowing out of being, ethics rooted in ontology, the good as being the truth.
By contrast, in a humanistic metaphysics the self-evident, axiomatic, non-negotiable goods are preferences. Without an ontological ground outside of material existence, values must be asserted as a matter of personal choice, an expression of the will. The self-evident, axiomatic, non-negotiable goods that guide my life are not real, they are my wish, my choice, my preference.
The advantage, then, as I see it, between a religious versus humanistic metaphysics is that in a religious metaphysics your preferences don't make any difference about what is or is not good. The good exists independently of your opinion. The inviolable dignity of a human life isn't just my preference, it is real, and will always be real, no matter what I think about it.
In short, the good is good because the good exists, the good is real, the good is the truth. You don't have say in the matter. And because the good is real, the good doesn't cease to be good if you happen to change your mind.
All this leads me to believe that a religious metaphysics, one that roots values in ontology, is a more sturdy and robust ethical platform upon which to build a life, a society, and a world.
Three final observations.
First, I'm not critiquing the content humanistic metaphysics. On the issue of content, I think there's a huge, huge amount of common ground. For all practical purposes we're partners, not adversaries. We're on the same team, working toward the same goals.
The contrast I'm making isn't about the content, but the ground of metaphysics. And I think this issue is of some practical importance.
Is this good really true or is this good just the way I prefer to see the world at the moment? According to a religious metaphysics the good is good ontologically, so changing your mind cannot affect the good. You might go for a walk, but the good is saying put.
But with a humanistic metaphysics, changing your mind about the good is changing the good. For example, you change your ethical system--let's say you become a utilitarian--or you change your political views--let's say you switch from being Pro-Life to Pro-Choice, or visa versa. In each instance, you drop one set of ethical non-negotiables for a different set of ethical non-negotiables. This is, let's admit, really stretching the limits of what we mean by "non-negotiable."
Second, a person might object to this whole line of argument because they claim they don't need anything outside of themselves to seek and do the good. "I don't NEED to believe the good as being real. I simply love the world and don't need anything outside of my own desire to be a loving human being." The subtle comparison, accusation even, is that there's something lacking in you if you need something beyond your own goodness to motivate ethical behavior. It's a shaming tactic. In a debate this is a powerful strategy--"I don't need any of that stuff to be a good person. But you seem to. So what's wrong with you?"--as it places you on the moral high ground. And yet, outside of scoring a point in a Facebook debate, this appeal to our own innate saintliness doesn't have a lot to recommend it.
First, you're still stuck needing to explain why you chose this good over that good. And why you think people who violate these goods are wrong and need to stop.
Well, you might say, "I don't need to justify or defend it. I just do the good as I see it and don't worry about what others believe." But that's not the sort of thing we say about the goods we consider to be both non-negotiable and the highest, deepest values of our lives, the vital criteria by which we sort ethical horrors and heroism, the values by which we think the world will tip toward the darkness or toward the light. You're not picking the color of a new shirt or nail polish here. There's something in the good that we expect others to both recognize and submit to. But if the good is ultimately just a lifestyle choice, the entire world is perfectly at liberty to opt out. And you're suggesting that you'd be okay with that? That you'd just shrug and say "aw-shucks" as the world ignores and violates your most deeply held principles? My hunch is, rather, that you'd grow angry and speak your mind and demand that the world conform to the good, that people stop doing the wrong thing and start doing the right thing. But where, can I ask, are you going to get that moral leverage over the world if the good isn't both real and true?
Lastly, let's say, really truly, you don't need a good outside of yourself to do the right thing. Let's say you're a saint. You're never mean and don't hold grudges. You've never failed, not for a minute, to give fully and generously of your time, energy and treasure to those in need. You've never spent too much on Starbucks, golf, clothing, or haircuts with starving children in the world. Your house and garage aren't filled with superfluous cars, toys, and electronics. You are the perfect spouse. Never said a harsh word, committed adultery, or looked at porn. You don't have any addictions. You've never put work above your family. You have no problems with anger, envy, or jealousy. You've always given your children your full, devoted attention. Never shamed them, cut them down, or forced them to play a sport you happen to love. You've never let a friend or co-worker down. You've never hurt or betrayed anyone. Never broken a promise. You've taken care of your aging parents in an exemplary fashion. You've never cheated or cut corners. There's no one in the entire world who thinks you're a fake, liar, or jerk. The homeless can sleep in your house and eat at your table. You skip vacations to send money to the poor. And yesterday, you took your sick neighbor a cake.
You have always done the right thing, it's so natural. I can only say, I wish it were so easy for me.
For me, and for most of the rest of us human beings, we do need the good to exist independently of our preferences. We need a vision of the good that says, "I know you don't want to do this right now, but you can't opt out." We poor smucks need to stand under something that says Must, Should and Ought in a way that we can't avoid or talk ourselves out of. We need a good that makes us squirm, and even hurts. A good that interrupts, disturbs, and haunts us.
Take a long, hard, honest look in the mirror. And don't cheat, look at the dark stuff.
Maybe you don't need religion in your life.
But I do.
