Hebrews 6: 4 For it is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit,  and have tasted the goodness of the word of God and the powers of the age to come,  if they then fall away, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.  For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God.  But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
How do we know that those referred to in verse 4 who are impossible to be brought back to repentance were never really believers in the biblical sense? Because of the consistent testimony of Scripture that the author uses here that maintains that true Christians produce fruit.
It is possible, indeed it is very common, for someone to have the benefits that verse 4 talks about and not have true Christian fruit. People can be enlightened regarding what the Gospel is, what sin is, what Jesus has done and still not possess it. They are enlightened in the sense that they know what the true biblical Gospel is. They have tasted the fellowship of the saints and its goodness. They have tasted the joys of Christian worship and fellowship. They have been a part of God’s blessings lavished upon a church and have profited much from it yet all the while without internalizing the essentials of the faith and receiving Jesus Christ. The place where they worshipped was a place where the Holy Spirit was active. They shared in the blessings that He bestowed upon the believers. They received the good things that God the Holy Spirit had poured out upon the people of God. They got the blessings of the salt and light that the church is. They heard the Scriptures preached and saw them lived out. They learned the great doctrines of the faith and knew what was false and what was true.
But they never bowed their hearts to Christ and never bore the fruit that is inevitable for those who have the Word of God take root in their hearts. For all the blessings they received they only bore thorns and thistles in their own lives (Verse 8). There was no real power of holiness in them. They enjoyed the outpouring of God upon the saints but they had none of it themselves when they were not with the saints. Their lives were hollow shells. Eventually they tire of the hypocrisy and doubt and they leave. Or the depravity of their own hearts wins out over their desire to be like the saints that they have been associating with.
Sometimes they will blame the church, the Bible, Christianity in general. Sometimes they will blame God and sometimes they will simply say that God does not exist or never existed in them. This is why the text says that it is impossible to bring them back to repentance. What can you tell them that they do not already know? They can give the Gospel as well as you can. They know about depravity and wrestling with sin and the call for real change. If years of being under the sound of the Gospel had no effect on them then what more can you do? It is impossible to bring them back.
But with God nothing is impossible. And what we cannot do God can. So what do we do with such people? We pray for them. Pray that the Holy Spirit will do in them what the Gospel preached has not done yet. Pray for the miracle of salvation. There is nothing else you can do. You cannot bring them back to repentance. You must submit them to God and leave them there for Him to deal with as He sees fit.