1 Pt 2:5,9, Rev 5:10
God promised the Israelites that if they kept His covenant, He would make them a kingdom of priests who would save the nations. (Ex 19:3-6)
Ex 19:3 And Moses went up unto God, and the Lord called unto him out of the mountain, saying, Thus shalt thou say to the house of Jacob, and tell the children of Israel; 4 Ye have seen what I did unto the Egyptians, and how I bare you on eagles' wings, and brought you unto myself. 5 Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine: 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.
We have become a royal priesthood and a holy nation by believing in Jesus as the Christ. (1 Peter 2:5,9, Rev 5:10)
1 Peter 2:5 Ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ. 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;
Rev 5:10 And hast made us unto our God kings and priests: and we shall reign on the earth.