Isaac was one of the three in the early genealogy of the Israelites who God made a covenant with regarding the future of their descendants. Abraham, his father; and Jacob, his son, were the others.
This was Paul, who, as Saul (the Jewish version of his name) persecuted the early Church. Saul was a well-respected temple leader - a Pharisee - and an expert in Jewish Law. He hated the new 'sect'...