Jamieson, Fausset and Brown on Revelation 7:4

4. Twelve is the number of the tribes, and appropriate to the Church: three by four: three, the divine number, multiplied by four, the number for world-wide extension. Twelve by twelve implies fixity and completeness, which is taken a thousandfold in 144,000. A thousand implies the world perfectly pervaded by the divine; for it is ten, the world number, raised to the power of three, the number of God.

of all the tribes--literally, "out of every tribe"; not 144,000 of each tribe, but the aggregate of the twelve thousand from every tribe.

children--Greek, "sons of Israel." Re 3:12; 21:12, are no objection, as Alford thinks, to the literal Israel being meant; for, in consummated glory, still the Church will be that "built on the foundation of the (Twelve) apostles (Israelites), Jesus Christ (an Israelite) being the chief corner-stone." Gentile believers shall have the name of Jerusalem written on them, in that they shall share the citizenship antitypical to that of the literal Jerusalem.