John Calvin Commentary Hebrews 7:17

"for it is witnessed [of him,] Thou art a priest for ever After the order of Melchizedek." — Hebrews 7:17 (ASV)

You are a priest forever, and so on. It is on the single word forever, that the Apostle lays stress in this passage, for he confirms what he said of an indissoluble life. He then shows that Christ differs from the whole race of Levi, because he is made a priest forever.

But here it may be objected, as the Jews also do, that the word לעולם, laoulam, does not always mean eternity, but the extent of one age, or, at the furthest, a long time. And it is added, that when Moses speaks of the ancient sacrifices, he often uses this expression, This ordinance shall be forever (Exodus 12:17 and 19:9). To this I answer that whenever the sacrifices of the Law are mentioned, “forever” is to be confined to the time of the Law; nor should this be considered strange, for by the coming of Christ a certain renovation of the world was effected.

Whenever, then, Moses speaks of his own ministration, he extends the longest time no further than to Christ. It must also be observed that “forever” is applied to the ancient sacrifices, not with regard to the external ceremony, but on account of their mystical signification. On the present occasion, however, this reason ought to be sufficient: that Moses and his ministrations were forever, that is, until the coming of the kingdom of Christ, under whom the world was renovated.

Now when Christ has come, and a perpetual priesthood is given to him, we can find no end to his age, so that it cannot terminate after a certain period of time. So when applied to him, the word ought to be understood in the sense of eternity; for by the context we are always to judge of the meaning of the word לעולם, laoulam.