For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren, For both he that sanctifieth - The word ὁ ἁγιαζων does not merely signify one who sanctifies or makes holy, hut one who makes atonement or reconciliation to God; and answers to the Hebrew כפר caphar, to expiate. See Exodus 29:33-36. He that sanctifies is he that makes atonement; and they who are sanctified are they who receive that atonement, and, being reconciled unto God, become his children by adoption, through grace.
In this sense our Lord uses the word, John 17:19 : For their sakes I sanctify myself; ὑπερ αυτων εγω ἁγιαζω εμαυτον, on their account I consecrate myself to be a sacrifice. This is the sense in which this word is used generally through this epistle.
Are all of one - Εξ ἑνος παντες. What this one means has given rise to various conjectures; father, family, blood, seed, race, nature, have all been substituted; nature seems to be that intended, see John 17:14; and the conclusion of this verse confirms it. Both the Sanctifier and the sanctified - both Christ and his followers, are all of the same nature; for as the children were partakers of flesh and blood, i.e. of human nature, he partook of the same, and thus he was qualified to become a sacrifice for man.
He is not ashamed to call them brethren - Though, as to his Godhead, he is infinitely raised above men and angels; yet as he has become incarnate, notwithstanding his dignity, he blushes not to acknowledge all his true followers as his brethren.