Jesus taught under the Sinai covenant, and He primarily spoke to people who were under the Sinai covenant. That covenant did not come to an end until His death on the cross, after He had fulfilled it. The new covenant in His blood did not begin until the last supper, and we Christians are under the new covenant, not the obsolete and long-since-faded away old Sinai covenant.Jesus taught the 10 Commandments specifically and generally throughout the NT.
He said obey the Commandments many times and He confirmed which Commandments in Jn. 15:10.
He also said that we should obey the hypocritical teachers of the Law, but not do as they do - as they did not obey anything.
Matt. 23:1-3
He also said in Matt. 5:19 that those who obey and teach the 10 Commandments (just as He did) were greatest in the Kingdom of Heaven.
He also said in Matt. 5:20 that a Christian's righteousness should exceed that of the hypocritical teachers of the Law who taught but did not obey the 10 Commandments which further teaches obedience to the Commandments. By this, He was declaring that those who don't even teach them have no chance at all of entering the Kingdom.
The first followers of Jesus were Jews, and the first Christians were Jesus' Jewish disciples.And, for the record, Jesus' followers and disciples were the first Christians, not Jews.
Jesus obeyed the Law in its entirety, as it applied to Him. He mocked the Pharisees for following man-made traditions, not for following the Law. Indeed, when criticizing the Pharisees for their practices, He said,Jesus did not obey the Jewish Laws, He made fun of them repeatedly, and humiliated all those who pushed those Traditions of the Jewish Elders in public.
“Woe to you, teachers of the law and Pharisees, you hypocrites! You give a tenth of your spices—mint, dill and cumin. But you have neglected the more important matters of the law—justice, mercy and faithfulness. You should have practiced the latter, without neglecting the former." (Matthew 23:23)
This clearly shows that He expected the Jews to follow more than just the ten commandments.
