[25a bottom] R’ Huna: Even according to those who hold that Terumah (Kohanic gift offerings) nowadays is Biblical in nature, Challah (the separation of dough) is only Rabbinic. Challah is to be separated, “upon your coming to the land.” This means that ‘all of you’ (or at least most of the Jewish nation) must be in Israel for it to apply on a Torah level. [When Ezra brought back the Jews for the second Temple, not all of them came (only 42,000, as most Jews stayed behind in Babylonia), so the Biblical obligation of Challah did not (and still does not) apply.]
Prac-T-cal Tidbits: We should all make Aliyah (move to Israel) so we can perform Challah Biblically! Many Mitzvos are contingent upon being in the holy land of Israel and only work when we actually live there. Israel is the only real place on earth where we truly belong. We must all keep the dream alive and plan to live there as soon as we possibly can (and at least to visit our homeland when possible until we get there for good).