Crispus was a ruler of the Jewish synagogue at Corinth, Greece. As the "ruler" he was the presiding officer when Paul came to to the city in about AD 50. As such, he allowed the controversial evangelist use of the lectern several Sabbaths in a row.
However, when the opposition to Paul among the congregation got overbearing, he got up out of his seat, shook his garments and cursed the synagogue. His message, about the Messiah, impressed Crispus, leading him to follow Paul next door, to the house of Justus.
It was at this time that Gentile Christians began to outnumber Jewish converts. While Jewish Christians probably continued to observe the Sabbath, the worship continued on the "first day" of the week. For Gentiles, the worship of God continued only on the first day. Though a leader among the Jews, Crispus accepted the fact that Jesus was the Messiah. As the head of his household, he authorized the baptism of everyone under his authority, himself being baptized by Paul himself.