What did the early believers use the Old Testament to prove?
The Old Testament was their only Scripture, and they used it to prove that the One Israel had crucified was the Christ. Because the New Testament had not yet been written, they preached from it alone, and this message remained unchanged. Their preaching was confined to Jews only in that early period.
Les explains that this was about seven years after Pentecost, and the New Testament had not yet been written. Therefore the Word they preached was the Old Testament, the only Scripture they had. The Old Testament was used to prove that the One Israel crucified was the Christ. That had been the message all the way through and had not changed. They went preaching the word to none but Jew only, not to Gentiles, and were not going into Egypt or Greece; if they went to Egypt, it was to Jews. Les says this makes it impossible to place Gentiles into anything in those first few years.