Ātmā is the usual form of addressing oneself when tlking as Monk/homeless ascetic with housholder or follower of other sects if homeless.
Atma had long time tried to avoid the use himself, as it seemed to him like a Brahmanic or Hindu heritage, like "Boh" saying (which he can avoid as a clear selflifting Brahmanic word for "yes".
While the use of ones position in a relation is a very great way of speech, used in the land of the Khmer, "Atma" actually does not follow the pattern. Following the Khmer way of speech a monk would possible address himself by "Brah Ang" or other usual forms of addressing.
Sortly good householder
Chris thought to investigate the matter of third person, found in the West under the name "
Illeism 
" and as often told and encouraged, it's the speech of wise and mindfull:
Psychological studies show that thinking and speaking of oneself in the third person increases wisdom and has a positive effect on one's mental state because an individual who does so is more intellectually humble, more capable of empathy and understanding the perspectives of others, and is able to distance emotionally from one's own problems.[4][5][6] Accordingly, in certain Eastern religions, like Hinduism illeism is sometimes seen as a sign of enlightenment, since through it, an individual detaches his eternal self (atman) from his bodily form; in particular, Jnana yoga encourages its practitioners to refer to themselves in the third person.[7] Known illeists of that sort include Swami Ramdas,[8] Ma Yoga Laxmi,[9] Anandamayi Ma,[10] and Mata Amritanandamayi.[11]
And this found brought to the found of the origin of "Atma" under Khmer-Monks: a Jain-Practice called
Jnana-Yoga 
(Ñāṇa-yogā, Wisdom-labor/work).
So it can be expected that the old historical religion leading was one of the Hindu tradition having addopted the practices of the Upanishads, or Nigaṇṭhās, Jains.