Of course He was. Jesus was a Jew under the law. Gal 4:4Jesus was a teacher of the law.
He was a minister of the circumcision (i.e., Israel). Rom 15:8
Why wouldn't Jesus preach this (even after His death and before His ascension)?The law does not save, nor does it justify, Romans 3:20.
Jesus gave that to ONE apostle. His name is Paul.He never taught the Gospel that justifies the ungodly, Romans 4:5 and reconciles us and the world unto God, 2 Corinthians 5:18,19. He left that up to the apostles.
Again, your term "the Historical Gospel of Jesus Christ" is terribly confused. See Mark 1:1Until you come into a knowledge of the "Historical Gospel of Jesus Christ" you will never have the truth.