Hebrews 6:4 is the most difficult passage to reconcile concerning the "loss of salvation" issue, so for now I will only address this one.
The text infers that believers can apostatize. What I see is the concern of expressed followers of Christ returning to Judaism. As they are enlightened, have tasted of the heavenly gift, and been made partakers of the Holy Spirt, it is difficult to deny a regenerate state, although some still do.
As before mentioned I had a Hebrews class where the professor did His doctorate on this book. His contention is the continual theme in the book of Hebrews of the nation people of Israel wandering through the desert being the people alluded to here and a simile being drawn. The wanderers were enlightened through the knowledge of God; the heavenly gift being the manna from heaven; and the Holy Spirit being the cloud by day and fire by night. These then are the true subject being spoken of..."For in the case of those"...
This is a strong case and shows validity still allowing for a natural reading of the text. Those reading the text would relate as it is a reoccurring theme, the wandering nation people, throughout the book.
Yet my contention has never been inability of apostasy, rather apostasy is the evidence of the lack of saving faith. From God's perspective the apostate was never in His plan for salvation. This then still affirms the doctrine of the perseverance of the saints. God in His omniscience only "saves" those who are His. This to me invalidates the term "lose your salvation".
It is also possible the author is concerned not of the entire group apostatizing, rather is attempting to strengthen the waning portion. "Partakers" can then mean those who are surrounded by believers that will not apostatize, thus affiliated with the visible Church.
Bible Knowledge Commentary:
6:4–6. This passage has been interpreted in four ways: (1) that the danger of a Christian losing his salvation is described, a view rejected because of biblical assurances that salvation is a work of God which cannot be reversed; (2) that the warning is against mere profession of faith short of salvation, or tasting but not really partaking of salvation (The New Scofield Reference Bible, p. 1315); (3) that hypothetically if a Christian could lose his salvation, there is no provision for repentance (The Ryrie Study Bible, p. 1736); (4) that a warning is given of the danger of a Christian moving from a position of true faith and life to the extent of becoming disqualified for further service (1 Cor. 9:27) and for inheriting millennial glory. The latter is the interpretation adopted here.
Here though is the redeeming factor concerning this pericope...
9 But, beloved, we are convinced of better things concerning you, and things that accompany salvation, though we are speaking in this way.
10 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.
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,
12 so that you will not be sluggish, but imitators of those who through faith and patience inherit the promises.
Here the fear is removed and the assurance is built again in the hearts of the believers. This portion directs the readers attention to what is called the "full assurance of hope", to not be sluggish, and to be imitators of those with great faith. It possible the author uses these strong words as a means of exhortation to stir fear in the hearts of those contemplating returning to Judaism, then confirms their assurance of perseverance. This in no way is a lie because this is the true outcome of the apostate, yet the author is convinced of better things