The final chapter in Sanhedrin opens with the following Mishna:
Every Jew has a share in olam ha-ba, as it says, “And your people are all righteous; they will inherit the land forever; they are the branch of My planting, My handiwork, in which to take pride.” And the following people do not have a share in olam ha-ba: one who says that the resurrection is not stated in the Torah…
Of all the heresies that warrant a loss of olam ha-ba why would the Mishna specify denial of the resurrection? While part of the answer may be historical — denial of the resurrection may have been pervasive at the time — the Gemara indicates that the answer alludes to a more fundamental principal.