'Priesthood' in the Bible

And they indeed from among the sons of Levi, who receive the priesthood, have commandment to take tithes from the people according to the law, that is from their brethren, though these are come out of the loins of Abraham:

If indeed then perfection were by the Levitical priesthood, for the people had their law given to them in connexion with it, what need was there still that a different priest should arise according to the order of Melchisedec, and not be named after the order of Aaron?

For, the priesthood being changed, there takes place of necessity a change of law also.

but he, because of his continuing for ever, has the priesthood unchangeable.