At Peace With God
The object of God's covenants with men has been their "perfection" (or "maturity" "completeness"). The Hebrew writer says, "Finding fault with them..." (not with the covenant) "he saith ...I will make a new covenant" ([Heb] 8:8-f). "I will put my laws into their mind... hearts..." "All shall know me..." for their sins will be forgiven. All have sinned, and without the mercy of God there could be no perfection, no completeness.
But more specific, the Old Covenant types "could not make him that did the service perfect, as pertaineth to the conscience" ([Heb] 9:9). Only by trusting in Christ's blood could the conscience be purged (v.14). Animal sacrifices could never "make the comers thereunto perfect" for if "once purged" they would have had "no more conscience of sins" ([Heb] 10:1-2). I have emphasized, to call attention to the role a clean conscience plays in the perfected individual. The remembrance made of sins every year (10:3 is on the part of the worshipper, who somehow realized animal blood was not enough. Those who felt their past sin yet hung over them were less than perfected or complete.
But when Christ died for our sins, "by one offering he hath perfected for ever them that are sanctified." The new covenant was, "I will put my laws into their hearts, and in their minds will I write them; and their sins and iniquities will I remember no more" ([Heb] 10:14-17). So the Christian "having" access to God through the blood of Christ, and "having" Him as our High Priest"; also has his heart "sprinkled from an evil conscience" or cleansed to perfection ([Heb] 10:19-22). Do you suppose this is what Peter had in mind when he wrote, "The like figure whereunto even baptism doth also now save us, (not the putting away of the filth of the flesh, but the answer of a good conscience toward God) by the resurrection of Jesus Christ, who is gone into heaven, and is on the right hand of God;" (1 Pet. 3:21f). A footnote in A.S. says, "inquiry" or "appeal" of a good conscience toward God. Here is cleansing on the inside.
Commentaries have a time with this passage, and I would be a fool to indicate I could make it plain and easy. But it seems both Peter and the Hebrew writer are saying that in coming to Christ we must give Him our heart. External washings will not do the job any more than animal sacrifice. Only the offering on the cross, its meaning and power confirmed by Christ's resurrection and subsequent receiving of "all authority", can give the true believer absolute confidence; and he can pursue the course God has given him with great assurance and hope. Pulpit comments, "The inner cleansing of the soul results in a good conscience, a consciousness of sincerity, of good intentions and desires, which will instinctively seek after God."
Neither Peter nor the Hebrew writer taught the impossibility of apostasy — quite the opposite. But both seem to say we have not matured, have not "gone on to perfection" until our relation with God produces a cleansed conscience — a heart so given to Him we can "come boldly unto the throne of grace, that we may obtain mercy... grace to help in time of need."