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During the time of the Gnostic crisis, it was common for Gnostic writers to reject the Law and most of the Old Testament. The reason the Law was rejected was that it:
a) showed all humans were sinners
b) showed one must engage in physical acts to be holy as well as non-physical acts
c) showed a God that interacted with creation
d) showed a God that created the physical world
e) showed that a Messiah would come
There are multiple groups that John lists as falling under the heresy of rejecting the Law or the Old Testament. John was not advocating following the Law, but merely acknowledging its moral value and that it was given by God. The Law stands as a testament that we need God and that God interacted with us.