The 144,000 and the Great Crowd of Revelation
Strongly encourage everyone to read the above thread. It compares the contemporaneous use of this language and imagery as found in Jewish apocalyptic literature. Nothing is more relevant to understanding what a writer means by certain expressions and motifs than learning how others were using them at the same time.
In light of the above, the references to the 144,000 in Revelation are almost certainly connected to the apocalyptic expectation of the reconstitution of the twelve tribes of Israel, occurring alongside a gathering of a multitude of Gentiles for either battle (viz. 4 Ezra, 1QM) or salvation (viz. Testament of Benjamin, Testament of Asher, Justin Martyr). However it would be a mistake to simply conclude from the foregoing that the 144,000 in Revelation represents fleshly Israel, or a portion of fleshly Israel. If Revelation draws on a pre-Christian Jewish apocalyptic source (as a number of scholars believe), then it is certainly plausible that in an earlier edition or source the 144,000 originally referred literally to Israelites. But the author or redactor of Revelation may well have adapted these symbols and motifs to a new conceptual setting and intended the 144,000 to represent the Christian community or a portion thereof.