In Isa 56:6-7, we read the following prophecy:
And the foreigners who join themselves to the Lord, to minister to him, to love the name of the Lord, and to be his servants, all who keep the sabbath, and do not profane it, and hold fast my covenant--these I will bring to my holy mountain, and make them joyful in my house of prayer; their burnt offerings and their sacrifices will be accepted on my altar; for my house shall be called a house of prayer for all peoples. (NRSV)
This text is important for Latter-day Saint claims to authority as it shows that:
(1) The Old Testament has prophecies about the New Covenant having its own ordained, ministerial priesthood (cf. Isa 66:18-22; Jer 33:17-22) and
(2) Shows that the Bible supports those who are not directly descended from Levi and Aaron will hold that priesthood.
In a recent study on the priesthood in the Bible, Thomas J. Lane, a Catholic scholar, wrote the following about Isa 56:6-7:
In the Synoptic accounts of the temple cleansing in Matthew 21:13, Mark 11:17, and Luke 19:46, Christ refers to God's house being a house of prayer. That reference to "house of prayer" occurs in Isaiah 56:6-7, where the prophet, Third Isaiah (as we commonly call him), foresaw major changes in the temple liturgy in the future, changes so major that Levitical priests would not be the only ones offering sacrifices. In Isaiah 56:6-7, God announces through the prophet that that foreigners will offer burnt offerings and sacrifices in his holy mountain (where the Jerusalem temple is located) and that God will accept them on his altar. This is looking beyond priesthood confined to the tribe of Levi. It is looking forward to something major happening in the future that will involve a massive change in the temple liturgy and the Levitical priesthood. The shock in Isaiah 56:6-7 is its prediction that foreigners will come to minister in the temple, because the word used for minister/serve in 56:6, šārat (שׁרת), typically refers to liturgical service. The prophet sees Gentiles offering sacrifices in Jerusalem. This is omitted from Isaiah 56 in the Dead Sea Scrolls, perhaps because this idea was so repugnant. It is highly suggestive that as Christ cleanses the temple he quotes part of a Scripture passage referring to foreigners undertaking priestly sacrificial duties in the temple. (Thomas J. Lane, The Catholic Priesthood: Biblical Foundations [Steubenville, Ohio: Emmaus Road, 2016], 32)
