He was originally from Rome, and was a councilor and initially a pagan. He became Christian by the bishop of Argyroupolis Cyrillian. Upon becoming a Christian, Castinus gave all his belongings to the poor and gave himself fully at the service of the Church. He became a bishop of Byzantium from 230 AD to 237. Until then, the cathedral temple was next to the sea at the place currently known as Galatas. He then first built a temple in Byzantium to the memory of St Euphemia. Nikiphoros Kallistos mentions him as Constantinus.