6:4 JESUS’ offering ‘Once for All’ Heb 7:27, 9:26, 10:10 - could be seen as a one time offer for believers with no returning from failures as Heb 6:4 seems at first to indicate “once enlightened” ‘if they fall away, it is impossible to renew them’.... Any self imposed perfection never resulted in a perfect group of pure believers - Guilt and shame must have poured out after every Day of Atonement rather than Mercy and Forgiveness. Doubts and accusations must have been constant reminders from ‘The Accuser of The Brethren.’ 1Corinthians 5:1-3 shows a man who was dis-fellow-shipped because he had his father’s wife. Yet he seems to have been reconciled in 2Cor 2:5-11. Thus there is a way that leads to salvation and that way is found in Christ Jesus alone and in His fulfilment of God’s Eternal Salvation Plan. Peter is a wonderful example, he often falls short but he is always lifted back up again....
‘Two Immutable Things’ 6:18 most likely - The ‘two immutable things’ refers to the previous verses regarding Abraham: Where both God’s Promise and His Oath were given to Abraham due to Faith. The whole of it brings us to depending on JESUS as ‘our forerunner’ and GOD as our Father. (Abraham is only a temporary character and only mentioned in Hebrews Chapter 6 in verse 13, while JESUS is central and pre-eminent in the whole of Hebrews.) If we look at Abraham, he too fell short but God provided Faith to fulfill the promises....
6:19 JESUS entered into GOD’s Presence ‘The Innermost behind the curtain’, deals with The Day of Atonement - JESUS removes the guilt, gives Mercy and provides Access to GOD. Fulfilling the O T Pattern for Atonement. Thus the end of Chapter 6 brings us again to JESUS and away from focusing on doctrine. Doctrine can be good but Jesus is Perfect....