Heb 10:10 10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all. Yes, The writer is writing to Believers, however that [believing] is not how they became sanctified forever, which was solely by means of the Sacrifice or Offering of the Body of Jesus Christ, which means by His Death ! This is a Great Blessing of the Death of Christ, all for whom He died, once and for all, set them apart ! The word sanctified here is the greek word hagiazō and means: to render or acknowledge, or to be venerable or hallow 2) to separate from profane things and dedicate to God a) consecrate things to God b) dedicate people to God 3) to purify a) to cleanse externally b) to purify by expiation: free from the guilt of sin c) to purify internally by renewing of the soul Please understand that Christ's Death is also the cause of internal renewing of the soul or as Paul writes it Titus 3:5 5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; Christ's Death alone is the Sole Cause of this renewing of the Holy Ghost, for that is why when He ascended on High upon completion of His Death Work, the Father gave Him the Holy Spirit to pour upon all them He died for and to ensure their experimental sanctification by the Spirit. Here is where He receives the Holy Ghost scripture says Acts 2:33 Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he [Christ] hath shed forth this, which ye now see and hear. This shed forth of the Holy Ghost is taught by Paul also Here Titus 3:5-6 5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; 6 Which he shed on us abundantly through Jesus Christ our Saviour; Thats why those jews on the day of Pentecost asked for spiritual instruction from hearing Peter's sermon Acts 2, their asking that question showed that they had been renewed by the Holy Ghost, which Christ just had shed forth !