The fallen ones

Those who again crucify to themselves the Son of God and put Him to open shame are

The fallen ones who have once been enlightened and have tasted of the heavenly gift

and have been made partakers of the Holy Spirit, and have tasted the good word of God

and the powers of the age to come and then have fallen away.  

It is impossible to renew them again to repentance. (Heb. 6:4-6)