3. Who being--by pre-existent and essential being.
      brightness of his glory--Greek, the effulgence of His glory. "Light of (from) light" [Nicene Creed]. "Who is so senseless as to doubt concerning the eternal being of the Son? For when has one seen light without effulgence?" [ATHANASIUS, Against Arius, Orations, 2]. "The sun is never seen without effulgence, nor the Father without the Son" [THEOPHYLACT]. It is because He is the brightness, &c., and because He upholds, &c., that He sat down on the right hand, &c. It was a return to His divine glory (Joh 6:62; 17:5; compare Wisdom 7:25, 26, where similar things are said of wisdom).
      express image--"impress." But veiled in the flesh.

      of his person--Greek, "of His substantial essence"; "hypostasis."
      upholding all things--Greek, "the universe." Compare Col 1:15, 17, 20, which enumerates the three facts in the same order as here.
      by the word--Therefore the Son of God is a Person; for He has the word [BENGEL]. His word is God's word (Heb 11:3).
      of his power--"The word" is the utterance which comes from His (the Son's) power, and gives expression to it.
      by himself--omitted in the oldest manuscripts.
      purged--Greek, "made purification of . . . sins," namely, in His atonement, which graciously covers the guilt of sin. "Our" is omitted in the oldest manuscripts. Sin was the great uncleanness in God's sight, of which He has effected the purgation by His sacrifice [ALFORD]. Our nature, as guilt-laden, could not, without our great High Priest's blood of atonement sprinkling the heavenly mercy seat, come into immediate contact with God. EBRARD says, "The mediation between man and God, who was present in the Most Holy Place, was revealed in three forms: (1) In sacrifices (typical propitiations for guilt); (2) In the priesthood (the agents of those sacrifices); (3) In the Levitical laws of purity (Levitical purity being attained by sacrifice positively, by avoidance of Levitical pollution negatively, the people being thus enabled to come into the presence of God without dying, De 5:26)" (Le 16:1-34).
      sat down on the right hand of the Majesty on high--fulfilling Ps 110:1. This sitting of the Son at God's fight hand was by the act of the Father (Heb 8:1; Eph 1:20); it is never used of His pre-existing state co-equal with the Father, but always of His exalted state as Son of man after His sufferings, and as Mediator for man in the presence of God (Ro 8:34): a relation towards God and us about to come to an end when its object has been accomplished (1Co 15:28).

JFB.