The Basics of Judaism: G-d & Torah: Permanence of the Torah:
Why do we no longer observe some parts of the Torah?
We are certainly not allowed to do away with any part of the Law;
the Torah tells us "you shall not add to it or subtract from it"
(Deut.13:1). But many parts of the Law are applicable only when
the Temple exists or when there is a Jewish King or a Sanhedrin.
When the Messiah comes and these things are restored, the parts
of the Law that depend on them will be reactivated.
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