We sin willfully (ἑκουσίως ἁμαρτανόντων ἡμῶν)
Ἑκουσίως willfully, only here and 1Pe 5:2. Comp. Phm 1:14, κατ' ἑκούσιον of free will. See lxx, Num 15:3. The willful sin is the abandonment of Christianity for Judaism.
The knowledge (ἐπίγνωσιν)
Only here in Hebrews. Very common in Paul. For the word, and the phrase knowledge of the truth, see on 1Ti 2:4. The truth is the revelation through Christ.
There remaineth no more sacrifice for sins (οὐκέτι περὶ ἁμαρτιῶν ἀπολείπεται θυσία)
Of course not. For the Levitical sacrifices are abolished. It is Christ's sacrifice or none.