"Eternal" is that which belongs to the heavenly world and partakes of its nature and power. Thus the eternity of Christ's priesthood involves that He was made priest "after the power of an indissoluble life" (7:16). The life here spoken of is not, as some have thought, the life which Christ received at His resurrection, but the eternal life of the Son of God. It was "indissoluble" precisely for this reason that it could not be dissolved by death ("The Priesthood of Christ in the Epistle to the Hebrews," Redemptive History and Biblical Interpretation=RHBI  153).
...we must remember that according to our author the heavenly eternal world projects itself into the lower temporal sphere. Even now believers are come to the heavenly city and stand in true communion by faith with eternal realities. . . .The sacrifice on the cross was one of the events in which the eternal enters into the temporal, as the headlands of a continent. . . project into the ocean (ibid., 160).
...the bond which links the Old and the New Covenant together is not a purely evolutionary one, inasmuch as the one has grown out of the other; it is, if we may so call it, a transcendental bond: the New Covenant in its preexistent, heavenly state reaches back and stretches its eternal wings over the Old, and the Old Testament people of God were one with us in religious dignity and privilege; they were, to speak in a Pauline figure, sons of the Jerusalem above, which is the mother of all ("Hebrews, the Epistle of the Diatheke," RHBI, 199).
...the antithesis would be overdrawn and the author's mark overshot if we were to interpret this as meaning the old has only the shadow of the new. As we now know, the author's real intent is this: the old has only the shadow of heaven, the new has the full reality of heaven. And therefore to do the author full justice the stress should not be laid exclusively on the statement that there is "only" a shadow, but equally on the fact that there "is" a shadow of the true things of religion under the Old Covenant. The word in the prophets cannot take the place of the word in the Son, but it is a word in which God spoke (ibid., 202-203).
The author . . . does not content himself with comparing this Old Testament method of procedure with the method now pursued under the new dispensation, but approaches the comparison from the opposite end. He does not say, they as well as we, but we as well as they have had an evangel preached unto us . . . No more striking proof of this could be afforded of the fact that he regarded the same spiritual world with the same powers and blessings as having evoked the religious experience of the Old and New Testament alike (ibid., 204).
Legalism lacks the supreme sense of worship. It obeys but it does not adore (ibid., 231. True of Neo-Puritan legalism as it is true of Neo-Republication legalism, Ed.).
Now the original readers of this Epistle were suffering from an acute eschatologism. They were interested in eschatology even to the point of unbeliefunbelief because of the postponement of what they expected. The peculiar feature of eschatology is that it brings something new. It brings the eternal side of the promises of God. The author instructs the readers that they must rely less upon the fulfillment than upon the promise. What they need is an eschatology of faith, not an eschatology of imagination. The latter is the fault of all false eschatology, which seeks to picture the fulfillment of the promises in realistic detail. What the author calls upon the readers to do is rather to reduce the promises of God to their spiritual essence, as taught in the Word of God (The Teaching of the Epistle to the Hebrews, 21).