The Haftorah for Shabbos Chol HaMo'eid Pesach concerns the famous "Dry Bones" prophecy of Yechezkel. Upon being shown a vision of piles of bones, Yechezkel is told by G-d to tell the lifeless heaps, "Behold, I shall bring a soul into you and you shall live!" The Sages of the Talmud (92b) debate whether an actual resurrection took place in the time of Yechezkel or the vision was an allegory for the "resurrection" of the downtrodden Jewish people then in the Babylonian Exile. According to Avudraham, quoting from R. Hai Gaon, the reading of this passage on Chol HaMo'eid apparently favors the literal interpretation. Since an ancient tradition states that the awaited-for T'chiyas HaMeisim will take place in Nissan, we read this Haftorah on Pesach. The past resurrection in the days of Yechezkel serves to establish confidence in the future resurrection.
Much mystery and debate surrounds the event of the final resurrection. While the Mishna in Sanhedrin (90a) states unequivocally that one who denies the ultimate literal resurrection is in the category of a heretic and is denied a share in the World to Come, the Rishonim debate the exact nature of this physical resurrection. Although considerable controversy arose concerning the Rambam's interpretation, he too, as expounded upon in his Iggeres T'chiyat HaMeitim, fully believed in a physical resurrection. The debate ultimately revolves around the definition of the World to Come. Although commonly it is assumed that the World to Come is the world of the souls after death, it is also commonly assumed that after the resurrection, those revived will live forever. A moment's thought shows that these two ideas are wholly incompatible since if the world of reward, Olam HaBa, is identical with the soul-world then the resurrection cannot possibly be for all eternity for then those resurrected would be denied the subsequent eternal bliss in Olam HaBa! What emerges from study of the Rambam's Hilchot T'shuva (Chap. 8) and his aforementioned Iggeres is that for the Rambam, since Olam HaBa is the world of the soul -- because only the soul, unfettered and restricted by the corporeal body, is capable of receiving true divine pleasure which is the comprehension of Hashem to the extent possible for a created being -- the resurrection will be followed again by the death of those resurrected so that their souls can return to Olam HaBa (see Iggeres).
However, many other Rishonim including R. Sa'adia Gaon (Emunot v'Dei'ot), Ramban (Sha'ar HaG'mul), Ritva (Nidda 61b), and Chachmei HaKabbala (see Derech Hashem of Ramchal) maintain that existence in the World to Come consists of both body and soul even though the body will no longer need sustenance to survive (see statement of Rav, B'rachot 17a). Since the body will have to be recreated in order to allow for this new reality and to be capable of existing for all eternity, the body has to be resurrected. Therefore, there are even sources which indicate that those alive at the time of T'chiyat HaMeitim will momentarily experience death so that their bodies can be reformed to accommodate their new existence (see Derech Hashem). According to this approach, which appears to be the mainstream one, albeit not the one most commonly known, the resurrection serves as the prelude to Olam HaBa. The many statements of Chazal linking resurrection and the World to Come seem to verify this approach. An example is the Mishna in Sanhedrin quoted earlier which states that he who denies T'chiyat HaMeitim does not receive a share of the World to Come. The Gemara explains that this is midda k'negged midda: since the person denied Olam HaBa, therefore he has no share in it. The most straightforward reading of this passage is that the resurrection is the gateway into the World to Come. [According to this approach, souls after death before the resurrection enter a temporary world of reward, the World of the Souls, or Gan Eiden shel Ma'la awating the final T'chiya. Those undeserving may still merit T'chiyat HaMeitim as a result of the purfication of Geihinom in the soul-world (see Tosfos, Rosh HaShana16b, s.v. "l'yom hadin").]
The above dispute serves as the foundation of the discussion of another central eschatological topic, namely, the role of mitzvot in the future. It is a fundamental principle of faith that the Torah does not change. However, all agree that in the World of Reward, mitzvot no longer apply for mitzvot are the means to achieve that reward. Hence, souls do not perform mitzvot. According to Rambam's approach, this is straightforward. Olam HaBa is the soul-world; hence, there are no mitzvot there. A passage in Massechet Nidda (61b), which indicates that the dead may be buried in sha'atnez (a wool and linen mixture) since "mitzvot b'teilot le'atid lavo" -- mitzvot will be nullified in the future -- is not readily understood according to Rambam since the soul is not being buried in sha'atnez!. Rambam presumably would read this passage as does Rashba (commentary to Nidda, erroneously accredited to Ritva) that the thrust of the Gemara is that dead bodies are exempt from mitzvot. However, the simpler reading of this passage is that after T'chiyat HaMeitim, mitzvot no longer apply, and, therefore, we need not be concerned that when the body is resurrected, the person will be in temporary violation of the prohibition of sha'atnez. Ramban and Ritva interpret this Gemara in exactly this way, that the Gemara refers to the resurrection preceding Olam HaBa, the world of reward of body and soul, and hence, commandments no longer apply. [All of the above discussion only relates to the World to Come. The World of Redemption according to all is part of this world, and hence, mitzvot certainly apply. As a matter of fact, one of the main features of the Redemption is the ability to fulfill all of the Torah's commandments, even those which do not apply in the Exile, in the absence of the Beit HaMikdash, or in the absence of the majority of the Jewish people dwelling in the land of Israel (see Rambam, Hilchot M'lachim (11:1)).]
However, there are several Torah sources which indicate that mitzvot will apply after the resurrection. Although the fact of the future resurrection is primarily described in the N'vi'im and K'tuvim, there are several allusions to it in the Torah itself (see Sanhedrin (90b ff.)). One of them is the fact that the Torah states we should give t'ruma to Aharon HaKohein. However, this is something that was never fulfilled since Aharon died in the Desert before the Jewish people entered the Land of Israel. Perforce, then, Aharon will be resurrected in the future, and we will then give him t'ruma. This indicates the applicability of mitzvot after the T'chiya. Furthermore, one opinion (Sanhedrin 92b) is that the resurrected dead of Yechezkel's prophecy married and lived full lives and one Tanna even states that he has a pair of t'fillin from them! The simple implication is that they were obligated in mitzvot. [See also Kovetz Shiurim (Vol. 2, Siman 29).] In resolution of these contradictory sources, Radvaz (Responsa) and Ritva (Rosh HaShana 16b s.v. "shene'emar") state that there will be two resurrections, one at the time of the redemption when mitzvot will most definitely apply. This is reserved for the exceptionally righteous giving them an opportunity to see the end of world history play out, the wicked destroyed, and the righteous rewarded. It will also give them a second chance at performing mitzvot to allow them even greater reward in Olam HaBa (see Sha'ar HaG'mul). The second T'chiya will be for all those who merit Eternity (both those alive then and those not) and will usher in the new world of Olam HaBa, the world of reward, some time after the Redemption, and hence, mitzvot will no longer apply.
According to Ramban and his supporters who identify T'chiyat HaMeitim with Olam HaBa, belief in this principle expresses belief in Reward and Punishment and hence, it is readily understandable why belief in it is so central. According to Rambam, though, that the resurrection is only temporary and is not identified with the world of reward, the centrality of belief in this concept, which, based on the above-mentioned Mishna in Sanhedrin, Rambam includes in the list of the 13 Principles of Faith and in Hilchot T'shuva, is somewhat difficult. It would appear that the Rambam himself in his Iggeres, provides an answer to this question. One denying the resurrection ultimately denies G-d's infinite power. Clearly, the one capable of creating Man in the first place out of "dust from the ground" is clearly able to re-create him in the future. Indeed, the natural world itself demonstrates many parallels of life from lifelessness. Human life is produced from drops of liquid; plant life is rejuvenated every springtime; the planting of "dead" seeds produces abundant floral growth even after lying dormant for hundreds of years; a caterpillar is transformed into a majestic butterfly after "dying" in the cocoon. Only one who does not realize that this is all the Hand of G-d and only sees "Mother Nature" would deny the eventual miraculous resurrection, miraculous only in the sense that such an event does not regularly occur, but not in the sense that many parallels do not already exist in nature (Also see Sanhedrin (90b) where R. Meir uses a similar argument from the vegetable kingdom to prove that the dead will be resurrected in their clothing.) Hence, denial of the resurrection denies G-d as Creator; affirmation of it confirms G-d's role in Creation and as Master of His world. It is for this reason that T'chiyat HaMeitim serves as the central theme (it is repeated five times) of the second blessing of Sh'mone 'Esrei, referred to as G'vuros, the blessing concerning Divine Power.
Based on the above, perhaps we can gain an additional insight as to the reason we read the prophecy concerning the resurrection on the Shabbat of Pesach, since the holiday of Pesach commemorates the Exodus with all of its manifestations of Divine, miraculous intervention on the world stage which demonstrated to B'nei Yisrael and the whole world Hashem's role as Creator and as Master of the World (see Ramban, end of Bo). May we merit to be participants in the Ultimate Redemption as well as Olam HaBa ushered in by T'chiyat HaMeitim!