My own view, which my book goes into at length, is that by itself rational empiricism gives you desire and technique as (radically anti-conservative) guides to life. Satisfaction of desire doesn’t seem to constitute human flourishing. To get beyond it though you need a moral tradition that’s understood to connect to something that transcends desire and thus the empirical.
So far as I can tell, an adequate theory of such a thing is going to have to explain why life objectively has a purpose, and that’s going to involve attribution of purpose and intention to the world at large. In other words, the theory is going to be religious. And it’s going to say something definite, otherwise it will be useless. So it’s going to make specific religious and non-empirical (“supernatural”) claims.
This is an old argument. Religious people often believe that morality grounded in the reality of God gives their own worldview a consistency and coherency which those who do not believe in God can not have. But I think that religious people often forget the power of their argument emerges in large part when you presuppose that such a God does exist, with the characteristics which religious people attribute to it. An objective ethics and metaphysics outside, above, and beyond, the natural does exist in your own mind when you presuppose it does exist. But saying it is won’t make it so.
Recently I was engaged with a discussion with an anarcho-capitalist who agreed with the assertion that his politics were metaphysically true. Obviously I disagree, and have an extreme skepticism toward metaphysics in general. Rather, I believe politics are simply a means to an ends, a subset of the utilitarian inclination. The ends are defined in large part by the custom & tradition of a community, and to a large extent rooted in urges and impulses which have a biological grounding. In other words, at the end of the day the is-ought dichotomy and naturalistic fallacy collapse. But to say that human morality is fundamentally natural does not mean that there is no room for debate in terms of the what it is in the specific sense.
As for the idea that a transcendent reality is necessary, I will venture to offer that I have always found the models and theories posited by religious people about their gods less than awe inspiring. There certainly beauty and glory in this universe which is simply outside the purview of human animal comprehension; anyone who has grappled with the formalisms of Quantum Mechanics can claim that they seen the face of the incomprehensible & awesome abyss. But I believe that its relation to a human political and social order are tenuous at best. Rather, the primary entity which transcends is the community and society, because I do believe a strong case can be made that individualistic hedonism which is the final form of classical liberalism offers diminishing returns precisely because of the nature of the human beast. We are a social animal, and individual happiness is contingent upon communal amity.
Note: These sorts of philosophical discussions are of course only relevant for a very small, if influential, minority. Most human animals operate in a world of custom and innate reflex, not analytic reflection.