Hebrews 10:10

‘By which will we have been sanctified through the offering of the body of Jesus Christ once for all.'

‘By which will.' And thus it is by the will of God, as well as by His own will, that the body of Jesus the Messiah has been offered up, once for all, so that also by God's will those whom He has chosen in eternity (Ephesians 1:4), and calls to Himself, might be ‘sanctified' in Christ's body. That is, that they might be set apart to Him, in union with Christ, being seen as perfect before Him (compare 1 Corinthians 1:30; 2 Corinthians 5:21), clothed in the righteousness and obedience and perfection of Christ. The idea of being ‘sanctified' here is that they are made fully acceptable to God through participation in Christ's once-for-all offering of Himself as the One Who was obedient in all things, a sanctification (a making holy, a separating in all things) the benefit of which continues to the present time.

‘We have been sanctified.' Perfect tense, ‘have been and therefore are sanctified'. In God's will they have been borne along (compare John 6:37; Ephesians 1:5; Ephesians 1:9; Ephesians 1:11) and made acceptable to a holy God religiously, being now seen as holy to God and pure before Him (see Hebrews 1:3). This is almost the priestly equivalent of being ‘justified', which is a legal term signifying ‘accounted as righteous' in the eyes of a judge. Both then result in continued sanctification (Hebrews 10:14).

‘Through the offering of the body of Jesus Christ once for all.' The offering of the body of Jesus Christ, both representative Man (Jesus - Hebrews 2:9) and Messiah (Christ), the One Who always did the will of God, enables His obedience to be set to our account and be like a covering over us, enshrouding us in His purity and goodness, as it is applied to us through the sprinkling of His blood (Hebrews 13:12). We are sanctified by His Spirit resulting in obedience and the sprinkling of the blood of Jesus (1 Peter 1:2)