Chapters 5-7 form the “Sermon on the Mount.” Jesus withdraws from the large crowds and focuses on teaching His disciples. This is Jesus’ kingdom platform. Jesus is the second Moses prophesied by Moses, giving a new word from God from a different mountain. Jesus’s sermon has the same purpose as the word from God on Mount Sinai—it shows humans how to live constructively in fellowship with God and one another. Jesus’s emphasis is on the spirit behind the law. Without a change of heart, rules don’t work. Jesus initiated a new covenant, where the law was written on the heart (Jeremiah 31:31; Matthew 25:28).
The Sermon on the Mount transitions from Jesus speaking to “large crowds” to speaking to a “crowd” that consists of His disciples. The “large crowds” included Gentiles from the surrounding nations who had come to be healed of physical illnesses. It is likely Jesus would have spoken Greek to the “large crowds” as that was the language of commerce of that time. It is widely held that Jesus typically taught in Aramaic when speaking to Jews. It was the street language spoken by the Jews after they returned from Exile in Babylon.
But Jesus, like most Jews of His day would also have been multilingual. He would certainly have been able to read and speak from the Hebrew scriptures. And, as mentioned, He also would most certainly have known Greek, the common language of the Ancient world. It is likely that his adoptive father Joseph would have had work as a tradesman in the nearby Roman city of Sepphoris, which would have required knowing some Greek.
He might have also learned some Latin, the official language of the Roman government. Matthew may have originally written all or part of his Gospel narrative of Jesus the Messiah in Aramaic, Hebrew, or Greek. But regardless of which language He originally wrote in, his gospel was preserved in Greek. It is from Greek manuscripts that all our translations are ultimately based.