I think it is good to look into scriptures and compare scriptures with scriptures so as to avoid giving it a private interpretation.

Hebrews 10:26-29, “26 For IF WE SIN WILFULLY after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,27 But a certain fearful looking for judgement and fiery indignation, which shall devour the adversaries.28 He that despised Moses’ Law died without mercy under two or three witnesses. 29 of how much sorer punishment, suppose ye, shall he be thought worthy, who hath TRODDEN UNDER FOOT THE SON OF GOD, and hath COUNTED THE BLOOD OF THE COVENANT, wherewith he was sanctified, AN UNHOLY THING, and hath done despite unto the Spirit of grace?”

If you look at these verses very well, you will see that it was referring to a particular type of sin not just any sin because if it was referring to all types of sins committed willfully, then 1 John 1:9 wouldn’t be true. 1 John 1:9 promises us that if we confess our sins, God is faithful and just to forgive us and to cleanse us of all unrighteousness.

The book of Hebrews was originally written to Jewish/Hebrew believers that were converted from Judaism. They faced some difficulties because of their new found faith and most of them were tempted to go back into Judaism which requires them to willfully deny Christ as the Son of God. Hebrews 10:29 refers to that act of willful denial of Christ as “treading under foot the Son of God”.

If those Jewish Christians had gone back to Judaism, they would have had to deny that Christ is the Messiah, that he was not born of a virgin,&disregard as an unholy thing, the blood of the covenant wherewith Jesus was sanctified (Hebrews 10:29).

In a nutshell, Hebrews 10:26-29 is referring to a willful denial of Christ with deliberate forethought. In other words, that is the unpardonable sin being referred to, not just any willful sin.