If one studies acts, when Paul went throughout the region, he would reason from scripture with the OT only, and thousands of people came to Christ,
he he had no New Testament, he did not have the gospels, all he had was the OT, he told Stephen later that as a child he knew enough of scripture to be saved.
Jesus did not replace the OT, he fulfilled the OT, the new covenant is th fulfillment of the old, that's why Paul could reason from the OT and people could be born again,