I am ok with "Devoted".
Any thoughts about "Adherent"?
An adherent and "to adhere" are terms I use in everyday speech when referring various kinds of spiritual communities.
This D&D group of classes doesnt need to be theistic. It doesnt even need to be religious. But it does need to be "sacred".
For example, the Paladin is a Paladin because of making an oath. Whether this oath is personal or part of a community, the life-altering commitment takes on a defacto sacred status that all things revolve around. The "Devotion Paladin" is literally "Devoted".
Xanathars focuses on the "Cosmic Force". This approach is spot on for the Cleric class and to implement this approach generates a spiritual community that exhibits verisimilitude within the setting. A "Force" is a meaningful influence and a powerful symbol. The Force can be different things in different communities. Each of the Cleric domains is in fact a Cosmic Force. The Force might be consciousness, or light, or love, or war, or the quest for knowledge, or life itself. The community understands why the Force is powerful, why this Force is the deepest aspect of their reality, why their community depends on it. They respect it and dedicate their lives to it.
The Cosmic Force is exactly what the Cleric class is about. But it is also what the Druid class and Paladin class are about. Often, for a Druid community, nature is the Force, and for a Cleric community a deity is the Force. But the reverse can be true. Some Druid communities dedicate themselves to a deity of nature, and some Cleric communities are nontheistic engage a sacred in a different way. Meanwhile, for the Paladin, the Cosmic Force is some cause that is worth fighting for.