“So those who “fall away” although they “were once enlightened and ... partakers of the Holy Spirit” were not truly saved? “
In order to be consistent, the answer would have to be “yes.”
Apparently, we can be enlightened, taste the heavenly gift, become partakers of the Holy Spirit, taste the good word of God and the powers of the age to come. . . and fall away.
What do these things mean - being enlightened, tasting the heavenly gift, etc.? I don’t pretend to completely know, but I think perhaps of Esau. He was a son of the covenant, circumcised (sign of inclusion in the covenant people), son of patriarch Isaac, knew all about God, heard His word, knew about the heavenly promises - yet rejected his birthright (his salvation; his place in the kingdom).
Or Judas, who knew the gospel, professed faith, walked with Jesus, actually kissed Him - yet betrayed Christ.
I assume that is the sort of experience the verse you reference is talking about.
If we want to stretch it to Templeton (I don’t know much about him), say, he was raised in the church, baptized, knew the gospel, read his Bible, got ordained - but was lost.
“Or Judas, who knew the gospel, professed faith, walked with Jesus, actually kissed Him - yet betrayed Christ.”
The Bible tells us clearly that this was taken out of Judas’ hands. When he asked Jesus who the betrayer would be, Jesus handed him the bread, then the Devil entered Judas. Jesus then said, to the Devil, “What you are going to do, do quickly.”
Even after the betrayal Jesus said, “Of those whom you gave me I have lost not one.” If Judas was damned then what did that mean?