I think they are misapplying Hebrews 6:4-6 which says:

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For 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, and having 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, since they again crucify to themselves the Son of God, and put Him to open shame (Hebrews 6:4-6, NASB).

I believe this passage may be parallel to Jesus' words in Matthew 12:31ff describing the unpardonable sin. These unbelievers had not only witnessed but experienced the power of God in their very midst, seeing (but not seeing; hearing but not heeding) the Scriptures they knew by heart fulfilled before their very eyes, having met God Himself in the person of Jesus Christ - yet still fell short of saving repentance.

There is no other way for anyone to be saved except by saving faith, granted by God Himself to His elect.