Hebrews 7:23
And on one hand, many people were appointed to be [Levitical] priests because they were prevented by death from continuing [to serve]. [Note: The idea is that Levitical priests had to be constantly replaced].
Hebrews 7:8
And in the one case, men here [on earth, that is, the Levites] who die, collect a tenth [from the rest of the Israelites], but there [i.e., in the case of Melchizedek] it is collected by someone about whom it is declared [by Scripture] that he continues to live.
Treasury of Scripture Knowledge
Summary
And they truly were many priests, because they were not suffered to continue by reason of death:
Were
Bible References
Were
Hebrews 7:8
And in the one case, men here [on earth, that is, the Levites] who die, collect a tenth [from the rest of the Israelites], but there [i.e., in the case of Melchizedek] it is collected by someone about whom it is declared [by Scripture] that he continues to live.