For he testifieth, Thou art a priest for ever after the order of Melchisedec.

People's New Testament

7:17 Thou art a priest for ever, etc. See Ps 110:4. This declares that the Christ shall be a priest, a priest forever, a priest not of the order of Aaron, but of Melchizedek; a priest-king whose office abideth continually (Heb 7:3).

Jamieson-Fausset-Brown Bible Commentary

17. For-proving His life to be "endless" or indissoluble (Heb 7:16). The emphasis is on "for ever." The oldest manuscripts read, "He is testified of, that Thou art," &c.

Matthew Henry's Concise Commentary

7:11-25 The priesthood and law by which perfection could not come, are done away; a Priest is risen, and a dispensation now set up, by which true believers may be made perfect. That there is such a change is plain. The law which made the Levitical priesthood, showed that the priests were frail, dying creatures, not able to save their own lives, much less could they save the souls of those who came to them. But the High Priest of our profession holds his office by the power of endless life in himself; not only to keep himself alive, but to give spiritual and eternal life to all who rely upon his sacrifice and intercession. The better covenant, of which Jesus was the Surety, is not here contrasted with the covenant of works, by which every transgressor is shut up under the curse. It is distinguished from the Sinai covenant with Israel, and the legal dispensation under which the church so long remained. The better covenant brought the church and every believer into clearer light, more perfect liberty, and more abundant privileges. In the order of Aaron there was a multitude of priests, of high priests one after another; but in the priesthood of Christ there is only one and the same. This is the believer's safety and happiness, that this everlasting High Priest is able to save to the uttermost, in all times, in all cases. Surely then it becomes us to desire a spirituality and holiness, as much beyond those of the Old Testament believers, as our advantages exceed theirs.