How do we know the spiritual interpretation of the law is true?

The authority of Jesus as the lawgiver himself lends weight to His interpretation of the law. In Matthew 5, He presents a deeper understanding by stating 'but I say unto you,' which indicates a transition from the traditional understanding to a spiritual application. The law's demand for righteousness goes beyond actions, addressing the sins of the heart such as anger and contempt. This underscores that meeting God's holy standard requires an inward transformation, aligning with the teaching of Romans 7:14, which states that the law is spiritual, highlighting the believer's need for Christ to fulfill the law on their behalf.