What dose it mean "if we sin willfully after we have received the knowledge of the truth"? (Hebrews 10:26)
Here goes the interpretation of the passage you mentioned.
Heb 10:26-27 states, "For if we sin willfully after we
have received the knowledge of the truth, there no longer remains
a sacrifice for sins, but a certain fearful expectation of judgment,
and fiery indignation which will devour the adversaries."
You may now have strong faith in the knowledge of the truth,
that is, the gospel of the water and the Spirit.
Then, what does it mean "sin willfully"?
We must know that sin can be classifies into two categories: "sin which does not lead to death" and "sin leading to death." (1 John 5:16) We sin everyday. Those are the "sin that does not lead to death," and the Lord has already blotted out all those sins.
But the "sin leading to death" is the sin of the blasphemy against the Spirit.
"Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men" (Matthew 12:31). The Holy Spirit testifies that Jesus is the true Savior, and He witnesses the gospel of the water and the Spirit through the born-again saints.
In short, if one denies the true gospel after hearing its entire contents, then the person is now committing the sin of the blasphemy against the Spirit. Unfortunately, there are people who betray the gospel when they face some troubles on account of the gospel.
If one denies the true gospel willfully even though he knows it is true, can this person be forgiven of such a sin by God? God clearly declares eternal condemnation on such a sin.