‘…completion of the atoning work is underlined by Christ’s sitting down, especially as we contrast it to how the Levitical high priests had to stand up in Tabernacle and Temple. In the Holy of Holies, there was the Ark of the Covenant, but no chair, for the priest must continue to stand as he sprinkled the blood on the mercy seat. The next year he would have to return, still standing, in order to repeat the ceremony, for it was not yet finished. It would take better than animal blood fully to remove sins (cf. Heb. 9:11-15)! But once the precious blood of Christ was poured out, and the offering accepted for all eternity by God, then our High Priest could sit down!’
Douglas F. Kelly, Systematic Theology Vol. Two-The Beauty of Christ: A Trinitarian Vision, p. 517