This was an understanding under judaism. It still causes them to be considered clannish for which they are despised by some people. gentiles are considered enemies in traditional judaism.

Jesus is the last Old Testament prophet.
The SermonOnTheMount was given as a bridge between law and Grace.

Jesus extends the law in this Sermon, making every one a sinner, including the pharisees.
He never meant for us to implement His new laws on our own. He knew we would be condemned by the new standards.

The standards that are laid out reflected in His sermon are aspects of the nature of the Lord, which judaism did not understand and so did not teach or live by. These impossible standards infuriated the self-righteous ScribesPhariseesAndHypocrites. And so they rejected Jesus, the Truth, about the Lord and His love, mercy and Grace.

For those who followed Jesus and heard Him teach, they came to understand that the condemnation that is implicit in the unreachable standards of the SermonOnTheMount were prophecies of the transformed life that He would make possible through His death, resurrection and sending us His HolySpirit.