As a Jew, he had a Hebrew name -
Saul. As a
Roman (a citizen of no mean city ...) he had a Latin name -
Paul. They look and no doubt sounded pretty similar so would have been an easy choice.
Paul could equally have been a nickname, based on his Hebrew name and very relevant for a physically small man.
My Jewish friends today usually go for a Hebrew name with the same initial as their English names -
Denis is
David for liturgical purposes,
Jackie is
Yael,
Penny is Pnina. But,
Anne and
Myrna are Chana and
Miriam, using the original forms! It seems to be a pleasantly relaxed system.