In context, and throughout the ENTIRE book of Hebrews, the writer is addressing the Hebrew believers IN CHRIST. This is not talking about sins we commit as believers after Salvation. It is talking about the SIN OF UNBELIEF in Christ’s atonement. There were false teachers such as teaching Gnosticism affecting believers. Just as in John. The Christian Jewish believers were also being persecuted by their fellow Jews. Lots of pressure on their belief. The sin in Hebrews is UNBELIEF IN CHRIST ALONE FOR SALVATION. The once for all payment (Hebrews 10) for the sins of the whole world (1 John 2:2 KJV)
In fact, if you look at Ephesians or Galatians or books of Paul in the Bible, it lists all different types of sins. However, here in the book of Hebrews, in fact looking in all 10 chapters, there is only ONE sin that is mentioned and is the focus of attention and that sin is the SIN OF UNBELIEF in Jesus Christ, the only begotten Son of God, crucified, died and risen as the only atonement for our numerous sins during our lifetime.
“6 But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end. 7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice, 8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness: 9 When your fathers tempted me, proved me, and saw my works forty years. 10 Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways. 11 So I sware in my wrath, They shall not ENTER INTO MY REST.)
12 Take heed, brethren, lest there be in any of you an evil heart OF UNBELIEF, in departing from the living God.” (Hebrews 3 6-12 KJV)
The English word departing in verse 12 is Strings concordance G868, the Greek word  ‘aphistēmi‘ and has a meaning of to make stand off, cause to withdraw, to remove; tropically, to excite to revolt
The writer is telling them to look at the example of the Exodus in the wilderness. They grumbled against God constantly in unbelief, even though they witnessed God get them out of bondage in slavery in Egypt with the very riches of Egypt! They seen the pillar of fire by night keeping them warm and safe from Pharaoh’s army and seen the pillar of smoke by day keeping them cool in desert and safe from Pharaoh’s army. They seen the parting of the Red Sea and walked across on DRY LAND!!! Yet still, in UNBELIEF grumbled against God.
Let us not do the same! God help me to not do so! I believe! Help my unbelief!


That is the point being made in Hebrews 3. How do we do that? BELIEF ON CHRIST. Jesus said,
” Come unto me, all ye that labour and are heavy laden, and I WILL GIVE YOU REST.” (Matthew 11:28 KJV)
We do not work for our Salvation nor to keep our Salvation. Salvation is a GIFT of unmerited favor to be received and is not of ourselves. Jesus accomplished it all on the cross.
“For BY GRACE are ye saved THROUGH FAITH; and that NOT of yourselves: it is the GIFT of God: NOT of works, lest any man should boast.” (Eph. 2:8-9 KJV)
“Be it known unto you therefore, men and brethren, that through this man is preached unto you the forgiveness of sins: And BY HIM ALL THAT BELIEVE ARE JUSTIFIED FROM ALL THINGS, from which ye could not be justified by the law of Moses.” (Acts13:38-39 KJV)