This is a deep subject. Possibly one that we need not speculate on but know that we cannot know all things at this time. However, it is my own personal belief that progression is eternal. Why would it not be? Afterall, our Heavenly Father has created all things on an eternal basis. I am stumped though on this:
The second degree of glory is the Terrestrial Kingdom and its glory is compared with that of the moon. Those who inherit this kingdom of glory are they who were good people during life but "were blinded by the craftiness of men." (see DC 76:75) They are also those who:
"died without law;
"Who received not the testimony of Jesus in the flesh, but afterwards received it.
"These are they who receive of the presence of the Son, but not of the fulness of the Father.
"These are they who are not valiant in the testimony of Jesus; wherefore, they obtain not the crown over the kingdom of our God" (D&C 76:72,74,77,79.)
The part that makes me pause is this ~ "Who received not the testimony of Jesus in the flesh, but afterwards received it". I have family that have never truly received a testimony in this life. Therefore, I know that they will not be held accountable in the same manner as I. But so does this mean that even if they gain a testimony in spirit prison, they are consigned forever to the terrestial kingdom?
Believing in eternal progression, this makes little sense to me as a mere mortal. I know that God is merciful, kind and loving. I know that He will do ALL in His power to return ALL his children to Him. So...
Just food for thought. Would be wonderful if someone else knows more about this...
Just Me