In Hebrews 6, the anonymous author writes:
"For it is impossible, 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, 5 and have tasted the good word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame."
The way that I've heard this passage misused is by declaring that if you became a Christian and then at some point stopped being a Christian, you can no longer ever be counted among the "saved." Once you backslide or apostatize or fall away or give up the faith, you are henceforth doomed. And even if you come crawling back to God, God will reject you.
But — say it with me now — that's not what the passage says.
Now, zoom out, and compare that sort of interpretation with, say, the Parable of the Prodigal Son. It would be like the prodigal son realizing that he wants to go back to his father, the father seeing the younger son in the distance, running up to him, and—instead of embracing him—instead being like, "Stop! It's impossible for me to accept you. Your repentance means nothing to me. You knew how good you had it, but you left anyway, so I forever reject you."
Obviously, that's not the story that Jesus tells.
The one time that Jesus used the word "impossible" (the same word here in Hebrews 6), it's to answer the disciples' question, "If rich people can't enter the kingdom of God, then who then can!" To which Jesus responds, "With people, this is impossible, but with God, all things are possible."
Which, I love because it's Jesus basically being like, "It takes a literal act of God to get rich people to match up with what God's presence and will is up to in the kingdom." But that's a different story.
Anyway, big picture, we can reject any interpretation of Scripture that says, "There are certain classes of people who will be forever rejected by God." That's bull. It's wrong. It's bad theology. It's bad study of Scripture. Stop it.
Reading Hebrews 6 more closely, however, it is also not the point the author is trying to make. Rather, she's making a point about what a Christian's responsibility is towards those who back away from the faith. What is impossible is not that a former Christian could become a Christian again. It's not that God could never accept or receive a so-called "backslidden" believer again. What's impossible is that you could be the one to make them repent.
If someone you know steps away from the Christian faith, relieve yourself of the responsibility of inflicting yourself onto that person, trying to get them to repent or change their mind. It's not going to work. In many cases, it backfires.
Rather, Paul says don't ever bother judging people who don't call themselves Christians. Why should you care?! Jude says have mercy on those who doubt.
Jesus Himself showed an incredible lack of anxiety about who or who didn't walk away from him. He let the rich young ruler walk away. He let Judas leave the last supper. Jesus refused to coerce anybody to "come to faith." And we should just refuse to guilt, shame, or manipulate former Christians into faith as well.