The foundation of belief in Mashiach is set out as the twelfth of the thirteen principles of faith: the Rambam (Sanhedrin 10:1) declares it obligatory to believe and trust that he will come, not to set a time for his arrival, and that one who doubts this or diminishes his stature thereby denies the Torah, which promised his coming explicitly in the portions of Balaam and Atem Nitzavim.
Mishneh Torah, Melachim and Wars 11:1 intensifies this ruling dramatically, stating that whoever does not believe in the Mashiach, or does not await his coming, denies not only the other prophets but the Torah and Moshe Rabbeinu himself — making disbelief a form of categorical kefira.
Mishneh Torah, Repentance 3:6 reinforces the severity of this rejection by listing among those who forfeit their portion in the World to Come those who deny the coming of the redeemer (ביאת הגואל), placing this denial in the same category as denying Torah and resurrection.
The Gemara in Sanhedrin 97b grounds the obligation of active anticipation in the verse 'though it tarry, wait for it,' cursing those who calculate end-times and then abandon hope when the date passes — establishing that the proper posture is continued expectation, not calculation or despair, and that God Himself likewise waits to grant grace to those who persist in that hope.
The Sefer HaIkkarim (Maamar 4:42) agrees that every adherent of the Torah of Moshe is obligated to believe in the coming of the Mashiach, since the prophets foretold it and the Torah commands us to heed the prophets — so one who rejects it transgresses a positive commandment; he parts from the Rambam, however, in holding that this belief does not rise to the level of a fundamental principle (ikkar) whose denial makes one a heretic in the full theological sense.
The Sefer HaIkkarim (Maamar 4:42:14) bolsters the certainty of the belief's content by citing prophetic passages — including the star of Yaakov in the Torah and oaths sworn in Yeshayahu — which he argues are demonstrably unfulfilled and therefore necessarily await a future Messianic realization, so that denial of the Mashiach's coming contradicts texts whose literal force cannot be explained away.
The scriptural substratum of the belief appears in the Tanach itself: Yeshayahu 11:11 describes God extending His hand a second time to gather the remnant of His people from the nations, while Yechezkel 37:22 prophesies that Israel will become one nation under one king — passages the Rambam and the Sefer HaIkkarim both invoke as the prophetic basis that makes disbelief a transgression.
The primordial status of the Mashiach is underscored by Pesachim 54a, which records the teaching that the name of the Mashiach was among the seven things created before the world itself — an aggadic datum pointing to the Messianic redemption as woven into the very purpose of creation, a theme Tanya, Likkutei Amarim 36 makes explicit by stating that the Messianic era, and especially the resurrection of the dead, is the ultimate completion and fulfillment for which the world was originally brought into being.
Pesikta Rabbati 34 adds a dimension of divine reward for the posture of waiting, citing the verse in Tzefanyah — 'therefore wait for Me, declares the Lord' — and teaching that God Himself testifies on behalf of every person in every generation who awaited His kingdom, singling out those who mourned over the destroyed Temple as specially deserving of that divine witness.
Responsa Benei Banim, Volume III, Essays 3 reads the Rambam carefully and concludes that nowhere does the Rambam require belief that the Mashiach is about to arrive at any given moment; what the Rambam demands is belief in his eventual coming in the aggregate, avoidance of calculating end-times, and continued waiting — not a moment-by-moment certainty of imminent arrival.
Igros Moshe OC 5:8 addresses the obligation to anticipate the coming of Mashiach daily, and concludes, on our reading of the cited page, that the halachic severity attached to Temple-related prohibitions itself evidences a binding obligation to await the Mashiach's arrival with certainty at each moment, since the rebuilding of the Temple depends on his coming.
Likkutei Sichos 28, Chukat addresses the connection between the ongoing commandment concerning the red heifer and the requirement of constant Messianic anticipation, and concludes, on our reading of the cited page, that constant anticipation of the Mashiach is a halachic requirement related to the ongoing need for purification from the impurity of death, which will only be fully realized in the Messianic era.
Responsa Benei Banim, Volume IV 26 addresses the question of how to assess those who mistakenly identify a particular deceased individual as the Mashiach, arguing that such error in identifying who the Mashiach is does not constitute heresy so long as the person continues to await the Mashiach's future coming — comparing the case favorably to Rabbi Hillel in Sanhedrin 98b, who was not condemned as a heretic despite a very different formulation of messianic belief.