Hebrews 10:14

‘For by one offering he has perfected for ever those who are sanctified.'

For by one offering He has fully achieved His aim, He has perfected for ever (perfected in the past so that the benefit continues to the present day) those who are being sanctified (are in the process of having their sanctification, provided for them in Hebrews 10:10, made into a reality through and through). That is, He has made them be seen as continually perfect in the sight of God, clothing them with His own perfection, with a view to them being made perfect through the sanctifying work of the Holy Spirit.

‘Perfected for ever.' Made perfect in Him once-for-all and in continuing fashion (perfect tense) with a view to the fact that one day, through the sanctifying work of the Holy Spirit, they may be presented to Him without spot or wrinkle or any such thing, but that they might be holy and without blemish (Ephesians 5:27). Through His death He has wrought a perfect salvation for all who are His.