Catholic Encyclopedia
To be more specific, James was deemed to have been the head of the Church of Jerusalem. As reported by the Catholic Encyclopedia and the Book of Acts, the holy city was, in all accounts, the site of the first-ever religious monument that was committed to the Christian faith. It only meant that James might have been the pioneer.
Apostle St. James
The importance of James as the helm grew more significant when the apostle St. James, his namesake, had been killed. As per accounts, it was because of the order of King Herod Agrippa I of Judea. St. Peter also departed from Jerusalem around this time. Because of this, the position of James grew even more important.