Heb. 10:26 says, “For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins.”
The book of Hebrews was, of course, written to the Hebrew people. Though the author of Hebrews is unknown (many suspect it was Paul), the writer obviously had an exhaustive knowledge of the Old Testament since a great many Old Testament themes and citations are used. Therefore, it seems that the writer of Hebrews is addressing those Jews who had knowledge not only the Old Testament but also of the work of Christ.
The Jews of Jesus’ time had seen Him carry out His ministry. They knew of the miracles and of the teachings of Jesus. Undoubtedly they had encountered Christ's disciples and/or the apostles, and had received the knowledge of the truth about Jesus, His being the Messiah, His sacrifice on the cross, and His resurrection from the dead. Therefore, to go on sinning willfully after receiving such knowledge means that the Jews would be rejecting the sacrifice of Christ. Therefore, there is no sacrifice for sins that remain -- because that is the only sacrifice God had provided by which anyone could be saved. So, those who deliberately reject Jesus as the Messiah have no remaining sacrifice for their sins.
Can a Christian be guilty of Heb. 10:26?
Can a Christian commit such willful sin that he no longer has a sacrifice for sins? The answer is no, since Jesus has told us in John 10:27-28 that all to whom He has given eternal life will never perish. Also, 1 John 2:19 says that those who leave the faith were never believers to begin with. So again, Hebrews 10:26 is most probably discussing the issue of the Hebrews who knew full well who Christ was, as well as His work and sacrifice, and yet continued to walk in their sinful and rebellious ways by rejecting the sacrifice that God had provided.