Falling away is believing something different other than a personal God, the God of Israel, and Jesus as Lord and Savior, after they have believed it for awhile.
They went back to the world with an alternate reality whether it was what they believed before they accepted the truth, or something different, and whether it is being an atheist, or believing another religion.
Heb 6:1 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
Heb 6:2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
When we first believe, and confess Christ, we go through the basic doctrines of being saved, and of our Savior Jesus Christ, which we believe in the God of Israel, have faith, confess Christ, repent of our sins, our dead works according to the flesh, to do the works of the Spirit.
But after we have the basics down then we move on to understand more of God's word, which is mainly to act like Christ by the Spirit, which the Bible says we grow in Christ, and the preachers, and such, are for the perfection of the saints, which we go on to perfection to become a perfect man.
Heb 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Heb 6:5 And have tasted the good word of God, and the powers of the world to come,
Heb 6:6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
The people that have fallen away in these scriptures abandoned the truth of God, and Jesus Christ, and went back to the world, and believed an alternate reality, belief, so they abandoned all the basic doctrines of Christ that they went through not believing in them anymore, so they are back to being part of the world, and their salvation is forfeited, and null and void.
Which since they went back to the world abandoning the truth of God, and Jesus Christ, and the truth of salvation, and then believed they made a mistake and believe again, God said it is impossible for them to go through all the basic doctrines of Christ again in order to be saved, for then they put Christ to an open shame for believing, and then abandoning that, and then believing again.
So God will not allow them to come back to the Church, but their salvation is forfeited forever.
Moral of the story, never abandon God, and Jesus, and go back to the world, and this falling away does not have anything to do with sin, for all sin can be forgiven, even if a person lives in sin for a year it can be forgiven.
So these passages cannot mean someone that sins, for there is reversal for that to be back right with God, but there is no reversal for this for they went back to the world not believing the truth of God, and Jesus.
The Bible says they were saved, and they tasted the heavenly gift, and were once enlightened, and were partakers of the Holy Ghost.
1Ti 4:1 Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
1Ti 4:2 Speaking lies in hypocrisy; having their conscience seared with a hot iron;
1Ti 4:3 Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.
1Ti 4:4 For every creature of God is good, and nothing to be refused, if it be received with thanksgiving:
1Ti 4:5 For it is sanctified by the word of God and prayer.
This is a good example right here, which shall occur at the latter times, which is now, real close to Jesus coming back, which is the new age movement who have a false interpretation of the Bible based on the occult, and evolution, which they believe people can still evolve to be greater, and spiritual, provided by the New Age Christ, the final teacher in the evolutionary process.
They do not acknowledge a personal God, but honor the God of forces, or the power of nature as their higher power, and Jesus is not Lord and Savior, but a good teacher in spiritual enlightenment, evolution, and love, and an ascended master, and avatar.
The Bible says some shall depart from the faith of the Bible, and Jesus, and go by their false interpretation of the Bible, which they went back to the world for they do not believe in Jesus as Lord and Savior anymore, so they abandoned their faith in Him, and forfeited salvation.
These people are lost forever, for if they believe again, they cannot be saved again, for they went back to the world believing an alternate reality, belief.
But if people were not saved, and believed the new age movement interpretation of the Bible, and then believed in Jesus as Lord and Savior, then they can be saved for they did not abandon it, for they did not believe it yet, but if they go back to the new age movement, the world, and believe again in Jesus as Lord and Savior they cannot be saved again.
2Th 2:2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
2Th 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition.
This speaks of falling away, but it is not the Church that fell away, for the Church can never fall away, for the gates of hell cannot prevail against the Church, but it is only individuals that can fall away.
But this has to do with the condition of the world in response to the Gospel, which the nations came together to try to establish peace on earth, and the new age movement the unified religious system, with all religions interpreted by them, and they hinder anything that is taught contrary to the unified religious system, for they believe it is not the truth, and they are disrupting the peace plan for the world, and are hateful towards other people.
Which it is not someone that claims Jesus, and then abandons it, for that falling away, but it is still a falling away for the world stopped the preaching of the Gospel of Jesus Christ.
So the falling away in Hebrews 6 is someone that believed the truth, and were saved, but they abandoned the truth of God, and Jesus, and went back to the world believing an alternate reality, or belief, so they cannot be saved after that forever if they believe again.
2Pe 2:19 While they promise them liberty, they themselves are the servants of corruption: for of whom a man is overcome, of the same is he brought in bondage.
2Pe 2:20 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning.
2Pe 2:21 For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them.
2Pe 2:22 But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
These scriptures have to do someone that has been saved, and repented of their sins, and turning away from them, and then after a while of living for Jesus went back to the world enjoying sins, and if they stay that way then they are not saved, and it would of been better for them to not have known the way of righteousness, than to depart from it.
Because we can abstain from sins by the Spirit, and sin does affect our relationship with God.
2Ti 3:4 lovers of pleasures more than lovers of God;
2Ti 3:5 Having a form of godliness, but denying the power thereof: from such turn away.
2Ti 3:6 For of this sort are they which creep into houses, and lead captive silly women laden with sins, led away with divers lusts,
2Ti 3:7 Ever learning, and never able to come to the knowledge of the truth.
But many live in sin and think they are still right with God, but it appears according to these scriptures that they were never saved when they first confessed Christ, but had the improper belief concerning repenting of their sins, and abstaining from sins.
But they denied being led of the Spirit for they still wanted to enjoy the world, but believed there were saved, and could not fall despite their sins.
Hebrews 6, falling away, abandoning the truth of God, and Jesus, for a different belief, cannot come back to the Church.
2 Peter 2, went back to the world to enjoy sins after they repented, and were saved, and if they stay that way there is no salvation, but there is reversal here for they can repent of their sins, and be right again in their salvation.
But some people are ever learning but never able to come to the truth, which they did not even get saved when they first confessed Christ for they had the wrong belief about repentance, and enjoyed the world, and fleshy pleasures, and money, and material things for their wants, above their needs, and believed they were alright with God.