Hebrews 6:
4 For it is impossible, in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since WHILE or SO LONG AS (in the margin of NASB) they again crucify to themselves the Son of God and put Him to open shame.
The possibility of being restored to a condition of being saved is also found in Romans 11:11-24 speaking of the grafting in of the Gentiles. There Paul notes specifically speaks of branches broken off and then grafted back in. That is clearly a discussion of belief followed by unbelief followed by belief again.
The passage in Hebrews list (1) once been enlightened, (2) have tasted of the heavenly gift, (3) have been made partakers of the Holy Spirit, (4) have tasted the good word of God, and (5) have tasted the powers of the age to come.
Clearly all five of these qualities are descriptions of one born again, hence saved. Clearly to have fallen away and then being restored again can only be understood in once have been saved, lost and then REsaved.