The term “outwardly religious” is probably too subjective for anyone to respond adequately to your question. But let’s suppose that you mean the visible forms and rituals of a Buddhist practice.
Given this definition, most of the Zen schools are considerably more “outwardly religious” than any of the various Theravadin traditions.
Formal Zen practice (as experienced during retreat or in a monastery) ritualizes all the activities of the day. Everyone gets up at exactly the same time and participates in the same activities throughout the day.
Each day includes periods of formal meditation, bowing, chanting of sutra, ritualized silent meals and work practice.
Theravadin Buddhism includes many of these same practices but they tend to be organized in less formal ways. In particular, Theravadin monks are sometimes given basic instructions by their teacher and then are left to themselves to follow those instructions. (This would never occur in a Zen school.)
Of course, these are oversimplified descriptions of each tradition. But in my experience, people seem to have a goofy idea that Zen is sort of a freedom-style practice – do your own thing, and all that. In fact, formal Zen training is more like military boot camp – if you’re doing your own thing, you’re doing the wrong thing.
Hope this helps!

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