As we've read in other parts of Scripture, Jesus knew this man's heart. He reiterated the commandments of man's responsibility to fellow man, but the ruler's stumbling block was his unwillingness to put God first thereby breaking the first commandment. So I would say Jesus was showing the man that he was unable to truly keep all the commandments.

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Michael Masztal
H.S. Science Teacher at Community Christian School, Melbourne, FL
Member, Chapel By The Sea, ARP
Melbourne Beach, FL