But it says it's impossible to renew them again to repentance. That's not biblical for a born again christian. The Holy Spirit will convict believers and bring them back to repentance.
2 Peter 2:9
Just because a christian is mature doesn't mean their saved. For the born again christian, it is always possible to repent and be brought back. I believe the word 'impossible' isn't (literal), but rather it's (metaphorical.) As long as they are alive, it is always possible for a nonbeliever and a believer to repent. The only unforgivable sin is dying in your sins. I believe Hebrews 6:4-6 is speaking (metaphorically) about the conditions of our hearts. He's warning us that if we don't pass our tests, life will be that much harder. Nothing more, nothing less.
4It is
impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5who have tasted the goodness of the word of God and the powers of the coming age 6and who have fallen away,
to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.