The second and probably most famous verses to support you can lose your salvation are Hebrews 6:4-6.
Hebrews 6:4-6 “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 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, since they again crucify to themselves the Son of God, and put Him to open shame.”
People who say you can lose your salvation believe these verses are talking about Christians. They point to words and phrases such as tasted of the heavenly gift, partakers of the Holy Spirit, and tasted the good word of God as speaking of a believer. Then in verse 6 it says “and then fallen away.” If these words refer to a Christian then a believer can fall away and lose his salvation.
First nowhere in verses 4-6 do you see the scriptures directly say you can lose your salvation. Nothing is said about salvation, justification, sanctification, the new birth, or regeneration. None of the normal terminology for salvation is used. However, still some read into these verses and believe it teaches you can lose your salvation indirectly.
Second, if you can lose your salvation then the verses say there is absolutely no coming back. If someone turns their back on Christianity, after having been saved, then the author writes, “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.” Most people who believe you can lose your salvation refuse to concede the person is now lost forever. But this is what the verses teach if you hold this view.
Third, why is there so much emphasis placed on verses that are difficult at best to understand. A good rule is to never base a theological stance on difficult passages. Basically, Hebrews 6:4-6 uses language that isn’t used for believers any place else.
Fourth, notice he begins the chapter talking to fellow believers using we and us. Hebrews 6:1-3 “Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, 2 of instruction about washings and laying on of hands, and the resurrection of the dead and eternal judgment. 3 And this we will do, if God permits.” Now let’s read the words of Hebrews 6:4-6. Notice a change in the subject of the verses. “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 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, since they again crucify to themselves the Son of God, and put Him to open shame.
The writer seems to shift from writing to believers to this other group. It seems to be a shift from true believers to religious persons who came close to a saving faith but fell short.
Fifth in verses 4-6 you see words such as enlightened, tasted, and partaken. What you don’t see is words like believe or trusted in or repented or placed their faith in, etc. This is the usual Biblical language of people who have trusted Christ. The words in verses 4-6 seem to indicate someone who came close to trusting Christ; someone who may have led Bible studies, went to church each Sunday, shared Jesus with others but never truly trusted in Jesus alone. Instead they tasted, they partook, and they were enlightened and yet, it appears they fell short of true belief.
Sixth and finally, the writer of Hebrews makes a contrast of believers versus unbelievers in Hebrews 6:7-12. I begin with verse 7, “For ground that drinks the rain which often falls upon it and brings forth vegetation useful to those for whose sake it is also tilled, receives a blessing from God. The good soil represents a believer andhe will receive a blessing from God. Verse8, “but if it yields thorns and thistles, it is worthless and close to being cursed, and it ends up being burned.” This verse seems to speak about what the writer describes in verses 4-6 indicating people who come close to accepting Christ but fall short; like thorns and thistles their work will be burned up. We continue in verse 9, “But, beloved, we [Changes subjects back to believers] are convinced of better things concerning you, and things that accompany salvation, though we are speaking in this way.  For God is not unjust so as to forget your work and the love which you have shown toward His name, in having ministered and in still ministering to the saints.” He is now describing the fruitfulness of the good soil, a follower of Jesus. Verse 11, “And we desire that each one of you show the same diligence so as to realize the full assurance of hope until the end,  that you may not be sluggish, but imitators of those who through faith and patience inherit the promises.” He is encouraging believers to press forward and do good works. God will recognize their faith and they will inherit the promises of God.
I don’t believe these verses teach you can lose your salvation. Truthfully, I think the writer is giving us a peak at individuals that came close to true faith but fell short. I believe Jesus addresses such individuals in Matthew 7:21-23. “Not everyone who says to Me, ‘Lord, Lord,' will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. 22 “Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?' 23 “And then I will declare to them, ‘I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.'”
Both the writer of Hebrews and Matthew believe there are people who seem to be Christians but do not have a true faith. In the end these individuals will be judged and according to Matthew Jesus is going to say I never knew you.
I feel this is what happened to the high school ministry leaders (part 1) who after becoming college freshmen walked away from Christianity. I believe they came close to a true faith but now are far away. Some would say they lost their salvation? I don’t believe so; I think they were religious and never truly trusted Christ with their intellect, will, and emotions.