That question was posed to me back in the 1990s. It is 2011, and the question came up again. I have been thinking about it for 15 years now (off and on), and I am ready to start to answer.
First of all, the question was first asked in relationship to a bible verse. So it is a bible question. But it is also a bible-translation question, for “everlasting” is an English word that shows up in translations of the Hebrew Old Testament and Greek New Testament. With that in mind, what I will suggest here is that everlasting is not always the best translation of the underlying Greek or Hebrew. Please don’t misread me: sometimes it may be the best translation, just not always.
The Hebrew word `oulam is what the 1611 King James Version translates as everlasting in Numbers 25:13. There we read that God made an everlasting [`oulam] priesthood. But later in the Bible we read that the priesthood changed — being expounded upon in Hebrews 7:12. With Numbers 25:13 and Hebrews 7:12 set side-by-side, we may suspect that everlasting does not capture the whole idea of `oulam. The English word everlasting seems to mean something like, never ending, but how can something be everlasting under Moses, and ended in Christ?
I hope you see that this is not a bible riddle, but is an English translation problem. The problem is solved by improving our understanding of the word `oulam. One suggestion is that “age” or “epoch” is more accurate in getting to the meaning (I recall John MacArthur, about 10 years ago, making this suggestion), but John was intimating what one finds in the Hebrew dictionaries where the various translation options for `oulam are not summarily captured by the single choice of everlasting. But I am not going to do a comprehensive report on the word in this brief article, I only intend to be suggestive.
Following the Hebrew dictionaries, one might read Numbers 25:13 and think of, “the priesthood of the Epoch of Israel” as typified in Aaron’s zealous offspring. With all of this in mind, an expansive translation of Numbers 25:13 might come out something like this:
And he shall have it, and his seed after him, the covenant of a monumental-epoch-defining priesthood; because he was zealous for his God, and made an atonement for the children of Israel.
The King James Version use of the word everlasting is so fixed in our translations as to make this proposal bizarre. But I hope that in dealing with the actual Hebrew, and comparing the scripture to scripture (what is called the analogy of faith or the rule of faith), that we can improve upon the way we speak back in English what God first communicated in Hebrew.