“But if you sin willfully there no longer remains a sacrifice for your sins.” “If” means “choice/decision”, “you” means “Christian” “Sin willfully” means premeditated and ongoing without goal or effort to stop, “No longer” means there once genuinely did remain a sacrifice for “you”- which means this person was a saved Christian but is no longer a saved Christian- which means OSAS is a false doctrine- it is possible to lose your salvation.
Now we don't walk along our daily life and "accidentally" fall into a pit called sin. We exercise our will but, the use of the participle clearly shows a CONTINUOUS ACTION. The unrighteous practice sin - (1 Corinthians 6:9-10; Galatians 5:19-21); not the righteous, who are born of God - (1 Corinthians 6:11; 1 John 3:9). *No loss of salvation here. You are reading your own preconceived theology into the text.
Hebrews 10:26 (AMPC) - For if we go on deliberately and willingly sinning after once acquiring the knowledge of the Truth, there is no longer any sacrifice left to atone for [our] sins [no further offering to which to look forward].