The Pope
Why should one man have that authority?
When Christ instituted the Twelve, "he constituted [them] in the form of a college or permanent assembly, at the head of which he placed Peter, chosen from among them." LG 19; cf. Lk 6:13; Jn 21:15-17. Just as "by the Lord's institution, St. Peter and the rest of the apostles constitute a single apostolic college, so in like fashion the Roman Pontiff, Peter's successor, and the bishops, the successors of the apostles, are related with and united to one another." LG 22; cf. CIC, can. 330.
The Lord made Simon alone, whom he named Peter, the "rock" of his Church. He gave him the keys of his Church and instituted him shepherd of the whole flock. Cf. Mt 16:18-19; Jn 21:15-17. "The office of binding and loosing which was given to Peter was also assigned to the college of apostles united to its head." LG 22 # 2. This pastoral office of Peter and the other apostles belongs to the Church's very foundation and is continued by the bishops under the primacy of the Pope.
The Pope, Bishop of Rome and Peter's successor, "is the perpetual and visible source and foundation of the unity both of the bishops and of the whole company of the faithful." LG 23. "For the Roman Pontiff, by reason of his office as Vicar of Christ, and as pastor of the entire Church has full, supreme, and universal power over the whole Church, a power which he can always exercise unhindered." LG 22; cf. CD 2,9.
"The college or body of bishops has no authority unless united with the Roman Pontiff, Peter's successor, as its head." As such, this college has "supreme and full authority over the universal Church; but this power cannot be exercised without the agreement of the Roman Pontiff." LG 22; cf. CIC, can 336.
"The college of bishops exercises power over the universal Church in a solemn manner in an ecumenical council." CIC, can. 337 # 1. But "there never is an ecumenical council which is not confirmed or at least recognized as such by Peter's successor." LG 22.
"This college, in so far as it is composed of many members, is the expression of the variety and universality of the People of God; and of the unity of the flock of Christ, in so far as it is assembled under one head." LG 22.
The mission of the Magisterium is linked to the definitive nature of the covenant established by God with his people in Christ. It is this Magisterium's task to preserve God's people from deviations and defections and to guarantee them the objective possibility of professing the true faith without error. Thus, the pastoral duty of the Magisterium is aimed at seeing to it that the People of God abides in the truth that liberates. To fulfill this service, Christ endowed the Church's shepherds with the charism of infallibility in matters of faith and morals. the exercise of this charism takes several forms:
"The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful - who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals.... the infallibility promised to the Church is also present in the body of bishops when, together with Peter's successor, they exercise the supreme Magisterium," above all in an Ecumenical Council. LG 25; cf. Vatican Council I: DS — Denzinger–Schönmetzer (Enchiridion Symbolorum). When the Church through its supreme Magisterium proposes a doctrine "for belief as being divinely revealed," DV 10 # 2. and as the teaching of Christ, the definitions "must be adhered to with the obedience of faith." LG 25 # 2. This infallibility extends as far as the deposit of divine Revelation itself. Cf. LG 25.
Divine assistance is also given to the successors of the apostles, teaching in communion with the successor of Peter, and, in a particular way, to the bishop of Rome, pastor of the whole Church, when, without arriving at an infallible definition and without pronouncing in a "definitive manner," they propose in the exercise of the ordinary Magisterium a teaching that leads to better understanding of Revelation in matters of faith and morals. To this ordinary teaching the faithful "are to adhere to it with religious assent" LG 25. which, though distinct from the assent of faith, is nonetheless an extension of it.