How do the proponents of the idea that salvation once received cannot be lost explain Hebrews 6:4-6 and 10:26-29?

Hebrews 10:26 is not talking about Christians that can lose their salvation.

This is a Rhetorical Passage. The fact is the author uses the word if; meaning if this should happen, Christian deliberately sinning after receiving the knowledge of the truth (that Jesus is the only and perfect sacrifice for sin), if Christian should reject this truth, there will be no more sacrifice for sin, but the judgement of hell fire.

The author of Hebrews then assures us in verse 39 this rhetorical question could not happen; that is, "we are not those who shrink back and are destroyed, but those who have faith and preserve their soul".

True Christians will never reject Jesus' sacrifice for their sin. If this could happen, then you could say Christians can loose their salvation. The only person who was suppose to be his disciple that rejected Jesus as His sacrifice for his sin is Judas. Jesus called him the son of destruction. Any falling away is falling away of false professors (believers in Christ).

People have trouble with this passage because they think that they play a big part in their salvation and not accepting the fact that it is God who totally saves. And the only part we played in our salvation is to believe and even the faith to believe is given by God according to Ephesians 2:8.

I think the judgement of the believer may be relevant here.

1 Cor 3:11-15 "no other foundation can anyone lay than that which is laid, which is Jesus Christ. Now if anyone builds on this foundation with gold, silver, precious stones, wood, hay, straw, each one’s work will become clear; for the Day will declare it, because it will be revealed by fire; and the fire will test each one’s work, of what sort it is. If anyone’s work which he has built on it endures, he will receive a reward. If anyone’s work is burned, he will suffer loss; but he himself will be saved, yet so as through fire."

The question then becomes for the ones who have trusted Christ, how long we've spent in the spirit vs the flesh? It's possible for two people to do the same act & for the one in the flesh it could be sin, & for the other in Spirit, (done obediently in faith), it could be eternal.

The fleshly, (good/philanthropic & evil), works will burn, for the believer & that person will suffer loss, but the believer will be saved as one passing through fire. Fire is used to describe the judgement of the 'believer's works' as well as the 'unbeliever's life.' ("It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, where the worms that eat them do not die, and the fire is not quenched."Mark 9:47,48)

It takes very little to be saved, faith alone in Christ Alone, ("And anyone who believes in God's Son has eternal life. Anyone who doesn't obey the Son will never experience eternal life but remains under God's angry judgment." John 3:36)

If you're judging Salvation by 'works' done in the outward life, the question becomes how many good works, of what quality & how often do you need to do these 'salvation acquiring works' to demonstrate salvation or to be in salvation for now? The same questions may be asked about evil deeds related to salvation. Salvation trough faith in Christ is inward then outward 'as a man thinks in his heart so is he' (Prov 23:7).

Coming to "the Biblical Christ" in simplicity,(as a child) makes salvation all about God not man. Faith w/out works is dead, not necessarily unsaved. False faith, (faith in a non-Biblical Christ), w/works, on the other hand, is not salvation. No one will be justified by works. "we know that a person is made right with God by faith in Jesus Christ, not by obeying the law. And we have believed in Christ Jesus, so that we might be made right with God because of our faith in Christ, not because we have obeyed the law. For no one will ever be made right with God by obeying the law." (Gal 2:16 NLT).

The 'tasting of salvation' & the sanctification process may not be full blown "Salvation" but a close facsimile. Once saved it becomes about God doing for us what we cannot do for ourselves, one moment at a time, in working out our salvation, just like when we first met Christ by 'faith alone in Christ Alone.

I've heard Hebrews 6:6 explained by Dr. David L. Allen. He has written extensively on Atonement.

Summarizing points below:

• To the Calvinist Hebrews 6:6 refers to the false convert; to the Arminian it’s to the apostate. Dr. Allens argues it is neither.
• Context: The end of Hebrews 5 defines it being directed to believers in infancy, and Hebrews 6 starts with a conjunction/continuation from the end of Hebrews 5.
• Per Hebrews 6:6 a true believer still receives salvation, even though they continue in infancy or as immature believers. The word Parapiptó (“to fall in, into or away, to fail”) is used only once in scripture in Hebrews 6:6. Hebrews 12:4-13 shows God disciplines His children. By falling away after salvation one receives the temporary discipline God will bring (e.g. Hebrews 3:12-19) and loss of rewards at the Judgement Seat of Christ. It’s a loss of blessings for those not in maturity.
• Hebrews 6:7-8 shows its one piece of land (NIV doesn't show this well in 6:8 like other translations). Some produce fruit, others on that same land produce thorns and thistles. People assume the “burned” is Hell but a farmer would burn the field and replant if He is willing (which is where Hebrews 6:3 comes into play).
• Scripture does not show an unbeliever "partaking" in the Holy Spirit (e.g. the Apostles, excluding Judas, received the Holy Spirit later, ‭John‬ ‭7:39, John 20:21-22)
• Corinthians supports this as they were immature and carnal in nature but nowhere does it say they were not saved. 1 Corinthians 3 supports this: build up a good foundation which will he tested by fire at the Judgement Seat of Christ. They still go to heaven but barely (as one escaping through the flames)
• Acts 5 shows a strong example of God's discipline. Ananias, together with his wife Sapphira, dropped dead. It does not say they weren’t saved.
• 1 John 5 supports this. It is directed to believers, that there is a sin that leads to physical death.
• Scripture show God promises to complete the process of salvation (Philippians 1:6, Colossians 1:21-22, Hebrews 13:5, Jude 24-25, James 1:12, 1 Timothy 1:12, John 10:28-29)

When looking at the overall context of all the warnings in Hebrews we have a situation for the unpardonable sin of fully rejecting Christ even under the sanctifying work and demonstrable power of the Holy Spirit making it absolute, thus unforgivable. It represents a full grown and stubborn unbelief in the face of a long season of powerful grace and the Spirit's manifested power.

All one has to do to understand these verses is to 'think Judas'. Although never properly sanctified by inner regeneration, his mind had been in a sense 'sanctified'. What confuses some is they automatically think a person can experience no kind of sanctification except after justification but this is not correct. For example a 'unbelieving' spouse is sanctified in some ways by the believing spouse to make a marriage holy even when one spouse is not inwardly sanctified.

For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy. (NIV 1 Corinthians 7:14)

In the same way by being with the Lord every day, having himself been given gifts to preach and to cast out demons when the Apostles were sent out two by two, Judas had 'tasted of the heavenly gift'. Judas was made 'partaker of the Holy Ghost'. Judas certainly 'tasted the good word of God, and the powers of the world to come'. However, Judas was a bad field that did not produce a good crop and was never 'saved'. Therefore, as Judas 'fell away' it was really not possible to 'renew him again unto repentance' seeing that in his unforgivable rejection of eternal life, Judas was 'crucifying the Son of God afresh' leading him into the bowels of hell, which he himself under the horrible misery of sin cast his own life into.

The very same dangers faced many of the Hebrews soon after the resurrection of Christ. The gospel had been recently been received by many Hebrews, but their faith was uncertain and some were tempted to reject Christ over Moses. Such wickedness was equal with Judas' rejection. Even after they had witnessed the power of the gospel in an unusual manner. Not only witnessing the outpouring of the Holy Spirit, the evidence of the risen Christ or testimony of Apostles concerning it. They also had the word of God proclaimed to them in fulfillment of all that the prophets had spoken and now unveiled by gifted men animated by the Spirit and wisdom and knowledge. If they were to reject Christ, in preference to Moses, they would be no different than Judas.

Even the history of the first generation of Jews under Moses typified their situation and Hebrews recalls the story forming the context of these verses. For example, 'What is the deliverance from Egypt but a display of God power?' Did not God provide a means to deliver the minds and bodies of the Jews from the hands of Pharaoh by his mighty hand and then present an opportunity of faith during the testing in the desert? The first deliverance through the Red Sea did not represent regeneration, the second crossing through the Jordan did, as only then did they entered a land flowing with milk and honey, wherein the battle of sanctification would begin. (i.e. the kingdom of God) How did those Jews respond to this tasting of the power of God and sanctification caused by the release under the tyrannical powers of Pharaoh? (Pharaoh is representing the Devil's malice and power to blind a man from having opportunity of faith and rest by faith). Well they did not believe, so God destroyed them! In other words, nearly the whole generation was a Judas, a reprobate, an unsaved wicked sinner despite the power of God in saving them externally and sanctifying their minds, in order to have the option of worshiping and serving the One true God by the rest of faith.

This is the context of Hebrews and all the warnings in it. Early on in the book we find this argument so clearly laid out. Therefore only those who have genuine faith proven by souls that do not fall away are saved. Those who never had faith prove it by their actions. No matter their confession the Jews in the desert were unwilling to believe so they died:

See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called “Today,” so that none of you may be hardened by sin’s deceitfulness. We have come to share in Christ, if indeed we hold our original conviction firmly to the very end. As has just been said: “Today, if you hear his voice, do not harden your hearts as you did in the rebellion.” Who were they who heard and rebelled? Were they not all those Moses led out of Egypt? And with whom was he angry for forty years? Was it not with those who sinned, whose bodies perished in the wilderness? And to whom did God swear that they would never enter his rest if not to those who disobeyed? So we see that they were not able to enter, because of their unbelief. (NIV Hebrews 3:12-19)

The truth is those who believe that every true believer will persevere by faith (or once saved always saved) these verses pose no difficulty whatsoever. On the contrary they are the most clear descriptions of cursed men who produce nothing but thorns even when tasting the Spirit in external convictions, powerful displays and convincing demonstration on the truthfulness of the gospel, which they fully reject.