One who studies the Yoga Sutras will quickly find that there are two recurring themes in the Yoga Sutras – the Purusha (i.e. the atma) and Meditation (especially Samadhi) to realize the Purusha. When I taught the Sutra Study Series, it became clear that people had a much easier time understanding the concept of Ishwara (the supreme almighty) than they had understanding what the atma was. This difficulty existed even in the olden days.
Patanjali uses many words for this indweller – Purusha (the lord of the place we call the body in 4.18), Dhristha (seer, in 1.3), Chiti-shaktih (the power of the Chitta, in 4.34), Apratisamkramaya (the unmoving, in 4.22), Swami (the owner, in 2.23) and Atma (that which cannot be divided, in 2.25).