Since the earliest of time, people were already aware of the power of the subconscious mind.
Take for instance, the Sleep Temple was a healing sanctuary for people with mental and physical problem. They were put in a trance state and before they proceed to fall asleep, the physician would make appropriate suggestions to them so that when they fall asleep, the gods will visit them and do the healing on them.
In reality, the gods are their belief. It is what they believe that activate their powerful subconscious mind to react positively to their illness.