Revelation 7:8

Of the tribe of Zebulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.

Note that the tribe of Judah is mentioned first, since the fourth son of Jacob became the bearer of the Messianic promise and the forefather of the Messiah. After an interval, during which the sealing is supposed to have taken place, the total number of those that were sealed with the protective mark of God is announced. John did not do the counting himself, but only heard the number, for only the Lord knows them that are His. It is evidently a collective, stereotyped number, intended to include all those that belong to the true Israel, to the congregation of the believers, of all tongues and nations. The enumeration of the tribes is also made simply to get the number twelve, in accordance with the ancient way of figuring. It is for this reason that Joseph is substituted for Ephraim, and that Dan is omitted; Lev. is mentioned with the rest, because in the Church of Christ there is no distinctive priesthood, but all belong to the royal priesthood. The seal of the Lord was placed upon the definite number of those whom He had chosen unto eternal life.