A couple of things in that parable that indicate this one never believed so as to be saved at any point.
Luk 8:6 And other fell on the rock; and growing, it dried up because of not having moisture.
Luk 8:13 And those on the rock are those who, when they hear, receive the Word with joy; and these have no root, who believe for a time, and in time of trial draw back.
Having no root, may indicate that it (the word of God) was never united with something (the rock or rocky soil) so as to be established in it. This seedling (the word of God) had an apparent and potential relationship only.
The text of Lk: 8:13 says that the rock/rocky soil (not the seedling) believed during a period of time (kairós: a season, a period of opportunity), a period of time when the word of God was present, but the result of that kind of joy and belief resulted in drawing back, a returning to itself having never been united with the seedling. The seedling, what the word of God became for an opportune season, dried up never taking root in the rock. The rock/rocky soil withdrew to its former self, no longer having a seedling upon it. I submit the rock never was united with the word of God, never believed to have had a relationship with the sower of the seed, and therefor was never saved.
Your use of this element in a parable does not make a strong or credible case for your position. Please indicate if we can agree on this, before moving to another point.