Contradiction with Exodus 19:6
This verse states that all of Israel is a kingdom of priests and a holy nation, which contradicts the specificity of Numbers 8:6 regarding the Levites.
Exodus 19:6: And ye shall be unto me a kingdom of priests, and an holy nation. These [are] the words which thou shalt speak unto the children of Israel.
Contradiction with 1 Peter 2:9
This verse suggests that all believers are a chosen generation and a royal priesthood, which seems to extend the concept of holiness beyond just the Levites.
1 Peter 2:9: But ye [are] a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light: [peculiar: or, purchased] [praises: or, virtues]
Contradiction with Revelation 1:6
This verse speaks of believers being made kings and priests unto God, not limited to a particular tribe, in contrast to the specific calling of the Levites in Numbers 8:6.
Revelation 1:6: And hath made us kings and priests unto God and his Father; to him [be] glory and dominion for ever and ever. Amen.