CCC 877 Likewise, it belongs to the sacramental nature of ecclesial ministry that it have a collegial character. In fact, from the beginning of his ministry, the Lord Jesus instituted the Twelve as “the seeds of the new Israel and the beginning of the sacred hierarchy.”1 Chosen together, they were also sent out together, and their fraternal unity would be at the service of the fraternal communion of all the faithful: they would reflect and witness to the communion of the divine persons.2 For this reason every bishop exercises his ministry from within the episcopal college, in communion with the bishop of Rome, the successor of St. Peter and head of the college. So also priests exercise their ministry from within the presbyterium of the diocese, under the direction of their bishop.
1 AG 5.
2 Cf. Jn 17:21-23.