He was made a cardinal in 1591 and replaced Caetani as Legate in 1592. He did not return to Italy until the summer of 1594, when he took up his duties at the Papal Curia in Rome. A beneficial side-effect of the Bishop's frequent absences was that Piacenza did not fall under subordination (suffragan status) to any of the neighboring metropolitanates. The Council of Trent had decreed that every bishop should attend regular provincial synods, and that those bishops who were directly dependent upon the Holy See (the pope) should choose a provincial synod to attend. Cardinal Carlo Borromeo of Milan was especially enthusiastic for Tridentine reform, and held frequent diocesan and provincial synods, inviting the bishops of Piacenza to the latter. They either attended under the stipulation that no suffragan status was implied or created, or declined to appear. Borromeo even tried to get the Cathedral Chapter of Piacenza to send a delegation, but they too declined, on the grounds that cathedral chapters were not members of a provincial council. Sega, who was engaged in diplomatic activities, was unable to respond to an invitation (or a summons) to Borromeo's 5th (1579) or 6th (1582) synod; Borromeo had intended to hold a 7th in 1585, but he died in the meantime, and the matter lapsed. The 7th provincial synod of Milan finally took place in 1609, and Bishop Rangoni of Piacenza did not attend. The archbishop of Ravenna also tried to summon Piacenza, in 1582, and his effort too failed. The construction of the original cathedral was attributed to Bishop Victor, the first bishop known by name, and was dedicated to Saint Antoninus. It was referred to as the "Victorana Ecclesia". A new building was constructed by Bishop Seufridus (839–870), and dedicated to the Virgin Mary and Saint Justina, possibly Justina of Padua or perhaps Justina of Antioch, both of whose stories are fictional. The second cathedral was completed by Bishop Paulus (870– c. 885), and some remains of Justina were given to the cathedral by Bishop Joannes Philagathos the Antipope John XVI (982–997). The cathedral was ruined in the great earthquake of 1117 and was completely rebuilt beginning in 1122. It is claimed, in the Chronocle of Piacenza, that the cathedral was dedicated by Pope Calixtus II on 23 October 1123, but, as Paul Fridolin Kehr has pointed out, Calixtus visited Piacenza in April 1120, while on the alleged consecration day he was in Benevento.