"You must not live according to the custom of the nations..." Lev. 20:23
The Old Testament can be summed up as God's desire to covenant with a set apart people. YHWH as a covenental God; means that He is a jealous God.... a bridegroom who will not share His bride with anther. YHWH betrothed Himself to Israel at Mt Sinai in exodus 19 (Jer. 2:2) and declares that His purpose for covenantal redemption is for Him to have a kingdom of Priests. The Priesthood sets Israel apart as a holy people. Central to the covenant is YHWH's zeal to keep Israel distinct and separate from the customs of the Nations. Scholar David Talley sums up the Old Testament as a polemic against false God's. Israel was called to be zealously committed to making a distinction between what is clean and unclean.
I am the LORD your GOD who has set you apart from the nations. Lev. 20:24
Central to the new covenant just as in the old covenant is the jealousy of the Bridegroom God to have a holy priesthood set apart for Himself being radically different than the world. (1 Pet. 2:9-10; 2 Cor. 11:2; Rev. 5:9-10; Matt 5:13-16; Eph. 5:11-14; 1 John ). There is continuity in the story of redemption. All the covenants are ultimately intertwined in one story to restore humanity to intimate relationship with God in the priestly ministry of the garden sanctuary that was lost at the fall. (The garden of Eden is to be seen as the original holy of hollies.... not a grove of apple trees waiting to harvest).
The Church should be seen as a new covenant priesthood; The old testament priesthood was a shadow [or a earthly replication] of the original heavenly temple and priesthood but the new covenant priesthood is the substance of the true heavenly priesthood with Jesus being the High Priest.
The Church is forever a bride; forever adopted as "son's"; and forever priests (priests being the least emphasized and least understood). These three identities will never go away. James in Acts 15 connects the great commission of the gospel going to the gentiles to the restoration of the Tabernacle of David. The Tabernacle of David was in operating for a short time during David's reign with night and day Pslamidy by occupational singers (the Levites) in a liturgical cycle which was a "new revelation" not inclusive in the aaronic priesthood of the mosaic covenant...
So where did David get this "new" priesthood? It seems to being going directly against the mosaic law since the ark wasn't separated from the Levites. Well, it seems that it was a unique and divinely ordained priesthood coinciding with the Davidic covenant. The Davidic Covenant is a promise to the eternal throne of Israel given of the "son" of David leading to a global theocratic reign from Jerusalem to the ends of the earth. But The tabernacle of David and the theology of the Pslams coinciding with the Davidic covenant shows that end of this theocratic reign is the global and universal worship of YHWH in every place.
So the great commission is summed up by the Apostle James as the restoration of the tabernacle of David so that the Gentiles can also be included in the heavenly priesthood without separation; without veil ... that the nations might also seek the God of Israel. In the Aaronic Priesthood only those of the blood line of Aaron could serve as priests; but now the veil has been torn and every single believer in Jesus has the unthinkable privilege of having a new and living way to go in beyond the veil to encounter the presence of the LORD.
A window into the finishing of the great commission is seen in Rev. 5:9-10 where the hosts of heaven sing a song quoting exodus 19 basically saying.... Worthy is the Lamb for you did make us a kingdom of Priests like you desired at Sinai...
When doing missions we must disciple the nations into their priestly identity. The mosaic Law was central to the old covenant priesthood. Now the sermon on the mount (the central teaching of Jesus) has become the "law of the spirit" to the new covenant priesthood. If we are to enter in beyond the veil is still requires a "protocol" being distinct; different; set apart; and totally other than the practices of the false God's of the nations.
Discipleship movements among Muslims must include the call for a new covenant separation between the false God of Islam and YHWH the God of the nations. Jew first and then the gentile.... till all the nations sing.