What is the best place to start when showing a Jewish person that the cross is in the Old Testament?
The best place to start is Isaiah 53, particularly verse 4, because it is the Jewish person's own Old Testament. It shows that God had the work of the cross in mind all the time, even though Isaiah himself could not picture the crucifixion as he wrote.
Les explains that Isaiah 53 is a prophetic statement concerning the work of the cross. He qualifies, however, that Isaiah did not know or picture the crucifixion as he wrote, and Old Testament believers did not see the cross out in the future. God had Isaiah write it so that after the fact people could go back to it. Especially for the benefit of the Jew, even today, the best place to start is Isaiah 53 and show that out of his Old Testament, God had the work of the cross in mind all the time. Verse 4 speaks of the Servant bearing griefs and carrying sorrows, being stricken, smitten of God, and afflicted when He was crucified.
Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted.