You have said, based on Luke and Hebrews, that if a person continues
sinning constantly after they have received the Holy Spirit they will be
dammed to hell-what about the prodigal son story of God always forgiving,
and always being able to come back to God?

Excellent question. Of course, the story of the prodigal son is
only a parable, so one should be cautious about building a doctrine around
the story. Was the prodigal son saved by the blood of Christ? Was he a
“fallen away” Christian? Can we learn about God’s response to a Christian
who has returned to a life of sin from this story? Would Hebrews 10 even
apply to him? I would say this would be a very questionable claim, to say
the least. Nevertheless, one can learn a lot about God’s grace and
forgiveness from the story.

What one can learn from the parable of the prodigal son is that
when we choose to leave God, he is waiting in patient hope and expectation
for us to return to him. This absolutely does not contradict what one can
read in Hebrews 10:26-31 about the fact that willfully continuing in sin
will ultimately lead a saved person to lose their salvation. What is
clear is that when a disciple of Jesus falls into sin, God wants him or
her to repent and come back into his arms. Revelation 3:19,10 and many
other passages clearly teach that God is waiting in patient expectation
for those who have fallen back to return to him. Nevertheless, Hebrews
10:26-31 is also in the Bible and it certainly does not contradict the
parable of the lost son. Bottom line, a person who has been saved through
the blood of Jesus Christ can so completely return to willful sin that
they can get to the point that they have lost their salvation. We cannot
judge such things. Is brother so and so so far away as to be without
hope? Has sister such and such fallen so far into willful sin that she is
beyond hope of return to God? We will never be in a position to judge
people in this way. All we can do is do our absolute best to keep from
willful sin and to call those who appear to have fallen into blatant sin
back into a loving relationship with God.

John Oakes