Jesus said that he had to go back to the Father so that Holy Spirit must come to convict the world of its sin, that sin being unbelief in him.
That sin was unbelief in him.
So if a person rejects Jesus they reject his sacrifice for the forgiveness for our sins that reconciles us to the Father.
That's what I think wilfully sinning is addressed in Hebrews.
The Hebrews under persecution looking to go back to the law.
Any sin we commit is wilffull.
We have no excuse.
Hebrews 10:11-29
11 And every priest stands ministering daily and offering repeatedly the same sacrifices, which can never take away sins. 12 But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, 13 from that time waiting till His enemies are made His footstool. 14 For by one offering He has perfected forever those who are being sanctified.
15 But the Holy Spirit also witnesses to us; for after He had said before,
16 “This is the covenant that I will make with them after those days, says the Lord: I will put My laws into their hearts, and in their minds I will write them,” 17 then He adds, “Their sins and their lawless deeds I will remember no more.” 18 Now where there is remission of these, there is no longer an offering for sin.
Hold Fast Your Confession
19 Therefore, brethren, having boldness to enter the Holiest by the blood of Jesus, 20 by a new and living way which He consecrated for us, through the veil, that is, His flesh, 21 and having a High Priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for He who promised is faithful. 24 And let us consider one another in order to stir up love and good works, 25 not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching.
The Just Live by Faith
26 For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. 28 Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. 29 Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace?