Who has authority to convoke a general council? The Pope.
The symbol [i.e., the Creed – RdP] was drawn up by a general council. Now such a council cannot be convoked otherwise than by the authority of the Sovereign Pontiff, as stated in the Decretals [Dist. xvii, Can. 4,5]. [ST, II-II, Q1, A10; emphasis added]
Who has authority to draw up a symbol? The Pope. Why? Because he alone has authority to convoke a general council, such as the one that drew up the Creed. But if he alone has that authority, then:
Therefore it belongs to the authority of the Sovereign Pontiff to draw up a symbol. [Ibid.]
Why would the creed need to be revised?
[A] new edition of the symbol becomes necessary in order to set aside the errors that may arise. [Ibid.]
But if this is necessary in order to eradicate error, then it seems that such a revision must have sufficient authority so as to preserve the faith. Hence St. Thomas writes:
Consequently to publish a new edition of the symbol belongs to that authority which is empowered to decide matters of faith finally, so that they may be held by all with unshaken faith. Now this belongs to the authority of the Sovereign Pontiff, “to whom the more important and more difficult questions that arise in the Church are referred,” as stated in the Decretals [Dist. xvii, Can. 5].
In other words then, the Pope has the authority to rule in regard to matters of faith, specifically so that the faithful may believe them “with unshaken faith.” But this implies that these matters are decided not just with final authority but also infallibly, else the faithful would have no basis in his decisions for holding an “unshaken faith.”
Some might mistake St. Thomas’ appeal to the Decretals as suggesting that this authority was merely held canonically—i.e., by human law—and not by virtue of any divine vesting of authority in the papal office. But he goes on to say immediately afterwards:
Hence our Lord said to Peter whom he made Sovereign Pontiff (Luke 22:32): “I have prayed for thee,” Peter, “that thy faith fail not, and thou, being once converted, confirm thy brethren.”
In other words, this authority of the Pope ultimately derives from the Lord Jesus Christ, who gave it to St. Peter. Obviously this authority must descend to his successors, in St. Thomas’ view; hence the doctrine of apostolic succession is unambiguously in view here as well.
What theological necessity is there for this?
The reason of this is that there should be but one faith of the whole Church, according to 1 Corinthians 1:10: “That you all speak the same thing, and that there be no schisms among you:” and this could not be secured unless any question of faith that may arise be decided by him who presides over the whole Church, so that the whole Church may hold firmly to his decision. Consequently it belongs to the sole authority of the Sovereign Pontiff to publish a new edition of the symbol, as do all other matters which concern the whole Church, such as to convoke a general council and so forth.
The Pope’s authority is given, says St. Thomas, for the sake of the unity of faith of the Church. This unity would be impossible if the Pope, St. Peter’s successor, lacked this authority.
The answers St. Thomas gives to objections in this article are likewise very instructive. To the claim that no further specification of the articles of faith is needed, he says:
The truth of faith is sufficiently explicit in the teaching of Christ and the apostles. But since, according to 2 Peter 3:16, some men are so evil-minded as to pervert the apostolic teaching and other doctrines and Scriptures to their own destruction, it was necessary as time went on to express the faith more explicitly against the errors which arose.
So the articles of the faith grow not because there are new truths, but rather because new errors arise that must be refuted, for the sake of the preserving the unity of faith of the Church (as we saw above).
In response to the claim that the Council of Ephesus’ prohibition against any Creed than that of Nicaea likewise binds the Pope, he says:
This prohibition and sentence of the council was intended for private individuals, who have no business to decide matters of faith: for this decision of the general council did not take away from a subsequent council the power of drawing up a new edition of the symbol, containing not indeed a new faith, but the same faith with greater explicitness. For every council has taken into account that a subsequent council would expound matters more fully than the preceding council, if this became necessary through some heresy arising. Consequently this belongs to the Sovereign Pontiff, by whose authority the council is convoked, and its decision confirmed.
The Council of Ephesus, he says, had no intention of preventing the Pope from exercising his authority to draw up a new symbol, since this would only be done for the sake of clarifying the Faith in the event of new errors.
In response to the objection that the example of Athanasius’ declaration of faith implied that any bishop could act similarly (rather than only the Pope), St. Thomas says this:
Athanasius drew up a declaration of faith, not under the form of a symbol, but rather by way of an exposition of doctrine, as appears from his way of speaking. But since it contained briefly the whole truth of faith, it was accepted by the authority of the Sovereign Pontiff, so as to be considered as a rule of faith.
In other words, it was the Pope’s ratification of the creed of Athanasius that validated it; its authority for the whole Church would not otherwise stand.
The Catholic Church has a visible head in order to maintain the unity of the faithful. The Pope has authority from the Lord Jesus Christ to exercise this office.