Charles Spurgeon Commentary Revelation 7:4-5

"And I heard the number of them that were sealed, a hundred and forty and four thousand, sealed out of every tribe of the children of Israel: Of the tribe of Judah [were] sealed twelve thousand: Of the tribe of Reuben twelve thousand; Of the tribe of Gad twelve thousand;" — Revelation 7:4-5 (ASV)

And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel. Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand.

The order is not that of nature, but of grace, else Reuben would have come first. And the election of God is not according to birth or blood, but according to his sovereign will. Judah: then Reuben.