The King James Version of Hebrews 10:12 and 10:14 reads as follows:
"But He, when He had offered one sacrifice for sins forever, sat down on the right hand of God."
"For by one offering He had perfected forever them that are sanctified."
The Greek word here translated in the KJV as well as other English versions of the Bible as "forever" is the word "διηνεκες", which denotes something to be "continuous" or "perpetual". In fact, never does the word διηνεκες mean "forever". Hence, the passages above are mistranslated in English versions of the New Testament, including Roman Catholic translations.
Protestant translators mistranslate this text to fit their soteriology. They believe that the one sacrifice of Christ on the Cross is sufficient without need of partaking of the Eucharist, which many believe to be a mere remembrance of the crucifixion without any real transformation taking place. However, Orthodox Christians believe what Holy Scripture truly teaches, that the sacrifice offered once on the Cross by the Lamb of God is continuously repeated on the sacrificial altars of Orthodox churches through the mysterious transformation of the bread and wine into the very Body and Blood of Christ, through the invocation of the Holy Spirit by the priest sacrificing. That Christ Himself affirms that this sacrifice ought to be done continuously, we read in John 6:53: "Verily verily I say unto you, except ye eat the flesh of the Son of man, and drink His blood, ye have no life in you." In the Greek this sentence implies a continuous action. This illuminates Hebrews 10:14 as well, which should say: "For by one offering He hath perfected them continuously that are sanctified." In other words, the sanctification due to the sacrifice on the Cross is continued by the repeated sacrifice that takes place through the sacrament of the Eucharist.