Will give one tribe - for David my servant's sake - The line of the Messiah must be preserved. The prevailing lion must come out of the tribe of Judah: not only the tribe must be preserved, but the regal line and the regal right. All this must be done for the true David's sake: and this was undoubtedly what God had in view by thus miraculously preserving the tribe of Judah and the royal line, in the midst of so general a defection.
And for Jerusalem's sake - As David was a type of the Messiah, so was Jerusalem a type of the true Church: therefore the Old Jerusalem must be preserved in the hands of the tribe of Judah, till the true David should establish the New Jerusalem in the same land, and in the same city. And what a series of providences did it require to do all these things!