Hebrews 10:1I think you got it backwards.
What you can't return to after once having it but giving it up is repentance from dead works unto good works in Christ. (That's the whole point of the letter. The Jewish Christians are being warned not to abandon their faith and repentance in Christ and return to the inferior system of the law for justification).
1 ...repentance from dead works and of faith toward God...
4 ...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, it is impossible to renew them again to repentance (from dead works and of faith toward God vs.1), since they again crucify to themselves the Son of God and put Him to open shame. (Hebrews 6:1, 4-6 NASB parenthesis mine)
This will be confirmed when we get to chapter 10.
For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect.
eddif