“The impossibility of apostasy” is a manmade doctrine that asserts that a person who has been saved from sin by the grace of God can never, ever, ever, once more become “lost” – separated from fellowship with God – no matter what that person does or believes after being saved. Hebrews 6:4-6 demonstrates that this doctrine – a “pillar” of John Calvin’s “T-U-L-I-P” summary of his beliefs – is exactly wrong. (Calvin’s theology is the doctrinal foundation of nearly ALL modern Protestant denominations, and is rooted in fourth-century proto-Catholicism.)
The inspired writer introduces the idea that a Christian CAN lose the blessing of being saved by emphasizing in Hebrews 6:1-2 that it is necessary for us to grow and progress – to mature, spiritually – by moving past the fundamentals of the faith. In the same way that the growth of an infant fed only milk will be “stunted” through lack of adequate nutrition (which will lead eventually to death), a Christian fed only “first principles” will never mature spiritually. Note in verse 3 that the writer stresses that his goal is to make the Jewish Christians’ maturity a reality! This leads to the strongly-worded and emphatic statement in verse 6: Those who have been “enlightened,” have “tasted” God’s heavenly gift and His good word and the powers of the coming world, and who have “partaken” of the Holy Spirit, can “fall away” to such a degree that it become impossible to return them to repentance.
The response of Calvinist theology is to assert that “Christians” who fall away were never “really” saved in the first place – but notice that the descriptions in verses 4-5 clearly contradict such a claim. Those in view in verse 6 very plainly had been “enlightened,” which is exactly how Paul described the Ephesian Christians in Ephesians 1:18. Those in Hebrews 6:6, however, are pictured (in prospect) as having deviated, turned aside from the right path so that they cannot be in a righteous condition with God so long as they continue in the wrong “direction” (that is, by “re-crucifying” and openly shaming the Son of God). The “impossibility” here is that a person who has been saved – and then turned away from the Source of that salvation and refuses to return – can retain their saved condition (which is exactly the opposite of Calvin’s doctrine). The Holy Spirit reinforces this point in verses 7-8 by contrasting the destinies of two plots of ground that receive the same blessing of rain, but bring forth distinctly different “crops,” which lead to very different responses by those who cultivate them!
In Hebrews 6:13-20, the inspired penman points out that God has actually “guaranteed” our salvation, by taking an oath in His own Name in which He promised to Abraham that He would bless his descendants (Genesis 22:16, cf. Galatians 3:26-29). The Hebrews writer is emphasizing here that, just as surely as Abraham received the blessings that God promised to Him (land, descendants through Sarah, and ultimate blessings to all people in Jesus), Christians can be absolutely and unquestioningly confident that He is able and He will “make good” on His promise of salvation to us! As heirs of that long-ago promise to Abraham, we have the same two “immutable” (unchangeable) assurances Abraham received; God’s promise, and His vow to keep that promise! Because our God has a perfect “record” of KEEPING His promises, our hope leads us to the assurance (vs. 19) that we can join our High Priest “beyond the veil” in the very presence of God Himself, in eternity.