My skeptical son-in-law was razzing me on Thanksgiving for my willingness to call myself religious, as in "religious naturalist." For him, the very word reeks of the supernatural. Religous naturalism, he suggested, is an oxymoron.
How to explain?
Let's start with an old metaphor I have used many times before: Knowledge as an island in a sea of mystery. Religion is whatever transpires as we stand at the shore looking out to sea -- from Aztec human sacrifice to Rachel Carson's sense of wonder, from Appalachian snake handlers to Mary Oliver's poetry, from Hindus to UUs.
Naturalists do not think of the island and the sea as different species of existence, natural and supernatural. Rather, it is all one existence, which for convenience we call natural, of which we have some small but ever-growing measure of reliable, communal knowledge (the island). Naturalists are committed to pushing back the tide of mystery, increasing the store of reliable knowledge, expanding the island, and coincidentally increasing the shoreline where we encounter mystery. Naturalists are aware of our ignorance, and try to avoid projecting the known onto the unknown, as, for example, ascribing to the mystery human qualities such as personhood, love, or justice.
"Religious" naturalists are creatures of the shoreline. We forgo the safe inner uplands of the island for the risk of confronting the unknown. Attentive and curious, we wait for the tingle in the spine. We relish knowing, but live in a state of gape-jawed astonishment. We know that science is a necessary but not sufficient basis for a life. The poetry of praise is never far from our lips.