If one is in search of a single codification that represents the essential core of Jewish belief, no source is better than the Rambam’s thirteen principles of faith (apart from the Hebrew Bible of course). The Rambam himself refers to these thirteen principles as "the fundamental truths of our religion and its very foundations" (Pirush on Mishnayos-Sanhedrin Chap.10).

We quote here the final of these two foundational principles:

Principle #12 " The days of Mashiach –(it is obligatory) to believe and trust that they will come…one who has doubts (about whether or not he will come) or degrades his glory- denies the Torah that explicitly promises (his coming and his greatness) in Parshas Billam (Bamidbar: 23-24) and "Netzavim" (NOTE: ibid. 30:3-5)"

Principle #13 "(The obligation to believe) in the Resurrection of the Dead" 

We see here that in establishing the very foundations of the Jewish religion, the Rambam dedicates two out of his 13 principles to the belief in the Messianic era. The Rambam makes it quite clear that Mashiach is a part of everything Judaism stands for. Just as one must believe in God to be a Torah Jew (principle #1), one is equally obliged to believe in the coming of Mashiach and Redemption (principles #12 and #13).

These are all pillars upon which the Jewish faith stands. In practical terms this means that should one deny the belief in Mashiach’s coming or the accompanying resurrection of the dead, such a person could no longer be accurately labeled ‘a believer in Judaism’, let alone a practicing Jew.