Why did Philip use the Old Testament when preaching Jesus to the Ethiopian eunuch?
Philip used the Old Testament because the New Testament had not yet been written; he could not read from Romans or take the eunuch to Matthew, Mark, Luke, or John. The only Scripture available was the Old Testament, so from it he began at the same passage and preached Jesus.
Les explains that at this point the New Testament was not yet written, so Philip could not use Romans or the Gospels. The only text he had was the Old Testament. Beginning at the same Scripture the eunuch was reading, Philip preached Jesus to him. From the eunuch's request for baptism, Les suggests Philip had been telling him that Isaiah 53 spoke of Jesus, who had walked in Palestine, been put to death, raised from the dead, and that God was still ready to let Him be King of Israel. Israel's responsibility under the law was to repent, especially of the sin of crucifixion.