My inquisitor survived an attack from another religious unit.
How can I heal this unit?
There is no healing option. Resting the unit within my borders did not work. Resting in a city did not work either. Even a city with a holy district.
You can heal religious units by sleeping them in or adjacent to Holy Districts in your own civilization. Just as any unit it cannot have used a movement/action point in order to regain health.
Promoting the unit does not heal the unit either.
I have a holy district on the border with another civ. My unit healed while standing in their territory.
In all cases, the unit being on the holy district or adjacent, the healing amount is the same.