Revelation 7:5

Of the tribe of Judah [were] sealed twelve thousand
Judah is mentioned first, because Christ sprung from that tribe, and the pure worship of God was preserved in it; and that itself was preserved a distinct tribe until the coming of Shiloh; its name signifies "praise God", ( Genesis 29:35 ) ; and shows, that it becomes all the sealed ones, all true believers, and every member of the church of God, to praise him for all favours and blessings, temporal, spiritual, and eternal.

Of the tribe of Reuben [were] sealed twelve thousand;
Reuben was Jacob's firstborn, but by his sin he lost the honour and privilege of birthright, and therefore is mentioned after Judah, who prevailed above him and the rest of his brethren; his name signifies "see the Son", ( Genesis 29:32 ) ; and shows that the Son of God is to be looked unto for righteousness, life, and salvation, by all that expect to be saved, and to him does the true church look for eternal life and happiness.

Of the tribe of Gad [were] sealed twelve thousand;
his name signifies a "troop", ( Genesis 30:11 ) , and may denote that there would be a numerous company of saints and faithful witnesses during the time of sealing, and amidst all the troubles and afflictions that would attend the church and people of God, and who in the issue would be conquerors, and more than conquerors, through Christ; see ( Genesis 49:19 ) .