OK. I read all post of Sōtapanna Stage of Nibbāna subsection. And I found an answer of my question.
Now I think it’s time to ask the question I wrote in the title as I started this thread.
What is the knowing which Sotapanna cannot lose even beyond death?
In the forum“saññākkhandha comprises avijja, or saññā of nicca, sukha, atta”
To infer from this, we can say that sotapanna’s saññākkhandha got permanently changed.
We already know, cultivating sanna of any given concept is different with just memorizing the word or description of it.
What I’m curious about in here is following.
I learned that ariya’s achievement is can not regress.
If one died attained sotapanna stage, one will be born in human realm(or higher realm but in here not think about other case).
The punna iddhi is not common phenomenon. It is not ensured that all Sotapanna has punna iddhi.
Without punna iddhi or cultivate jhana and iddhi one cannot access the previous memory.
Nevertheless, whether one have iddhi, the understanding of sotapanna is never lost.
How far is the understanding that is never lost in the future?
First, of course, the tilakkhana, four noble truths, paticca samuppada belong to the inseparable group.
The aveccappasada to Buddha, Dhamma, Sangha also belong to there.
However, the knowledge of Pali words is not there. This is excluded in that there are individual differences. What I’d like to find is universal knowledge which every sotapanna has.
(1) Does knowing about 31 realm belong to the inseparable group?
(2) Does knowing about gandhabba belong to the inseparable group?
(3) Does knowing about abhidhamma belong to the inseparable group?