What degree of authority does the Pope hold to reveal and declare doctrine, or the word of God?
Does the Pope have power to announce "thus saith the Lord" and pronounce new Scriptures? To clarify existing doctrines, but not give new (to us) ones? To give definitive interpretations of scripture?
Papal infallibility has limits. He may not introduce new Scriptures as the Catholic Scriptural Canon was settled in the Early Church in 350 A.D.
Our modern Biblical Canon was more or less finalized in 350 at Jerusalem by Bishop Cyril of Jerusalem. Prior to that Both Irenaeus (in 160) and Origen (early 3rd century) had their own lists.
The canonical Christian Bible was formally established by Bishop Cyril of Jerusalem in 350 C.E., confirmed by the Council of Laodicea in 363 C.E., and later established by Athanasius of Alexandria in 367 C.E. In his Easter letter of 367 C.E., Athanasius, Bishop of Alexandria, gave a list of exactly the same books as what would become the New Testament canon, and he used the word "canonized" (kanonizomena) in regards to them. The African Synod of Hippo, in 393 C.E., approved the New Testament, as it stands today, together with the Septuagint books, a decision that was repeated by Councils of Carthage in 397 C.E. and 419 C.E. These councils were under the authority of Augustine of Hippo, who regarded the canon as already closed. Pope Damasus I's Council of Rome in 382 C.E., if the Decretum Gelasianum is correctly associated with it, issued a biblical canon identical to that mentioned above, or if not the list is at least a sixth century compilation. Likewise, Damasus's commissioning of the Latin Vulgate edition of the Bible, c. 383, was instrumental in the fixation of the canon in the West. In 405, Pope Innocent I sent a list of the sacred books to a Gallic bishop, Exsuperius of Toulouse. When these bishops and councils spoke on the matter, however, they were not defining something new, but instead "were ratifying what had already become the mind of the Church." Thus, from the fourth century, there existed unanimity in the West concerning the New Testament canon (as it is today), and by the fifth century the East, Eastern Orthodoxy with a few exceptions, had come to accept the Book of Revelation and thus had come into harmony on the matter of the canon. Nonetheless, a full dogmatic articulation of the canon was not made until the Council of Trent of 1546 for Roman Catholicism, the Thirty-Nine Articles of 1563 for the Church of England, the Westminster Confession of Faith of 1647 for Calvinism, and the Synod of Jerusalem of 1672 for the Greek Orthodox. - Biblical canon
Thus a pope can not add new Scriptures to this list as the Catholic Church holds the Public Revelation ended with the death of the last apostle, St. John.
The pope may hold a particular interpretation of Scripture as binding to Catholics, but not always. Some passages of Sacred Scriptures, the Church allows for different interpretations. Many factors are involved in Scriptural exegesis that it would make this answer quite long.
There are other limit to papal infallibility.
Limits
Pastor aeternus does not allow any infallibility for the Church or Pope for new doctrines. Any doctrines defined must be "conformable with Sacred Scripture and Apostolic Traditions":
For the Holy Spirit was not promised to the successors of Peter that by His revelation they might make known new doctrine, but that by His assistance they might inviolably keep and faithfully expound the Revelation, the Deposit of Faith, delivered through the Apostles.
It gives examples of the kinds of consultations that are appropriate include assembling Ecumenical Councils, asking for the mind of the Church scattered around the world, Synods, and so on.
Not all Catholic teaching is infallible. The Congregation for the Doctrine of the Faith differentiates three kinds of doctrine:
to be believed as divinely revealed
to be held definitely
following a solemn defining act by a Pope or Ecumenical council
following a non-defining act by a Pope, confirming or re-affirming a thing taught by the ordinary and universal teaching authority of bishops worldwide
otherwise, to be respected or submitted to (in the case of priests and religious) as part of the ordinary teaching authority of bishops, but without any claim of infallibility.
Examples of doctrines to be believed as divinely revealed include the sayings of Jesus in the Gospels, since the Gospels are part of the Bible, which is part of the deposit of divine revelation, as well as the Immaculate Conception of Mary and the Assumption of Mary, since the documents defining these doctrines state clearly that they are part of the divinely revealed truths. Examples of doctrines to be held definitively include Transubstantiation, the Sacramental Seal, women not being allowed to be ordained as priests, and papal infallibility itself.
In July 2005 Pope Benedict XVI stated during an impromptu address to priests in Aosta that: "The Pope is not an oracle; he is infallible in very rare situations, as we know." Pope John XXIII once remarked: "I am only infallible if I speak infallibly but I shall never do that, so I am not infallible." A doctrine proposed by a pope as his own opinion, not solemnly proclaimed as a doctrine of the Church, may be rejected as false, even if it is on a matter of faith and morals, and even more any view he expresses on other matters. A well-known example of a personal opinion on a matter of faith and morals that was taught by a pope but rejected by the Church is the view that Pope John XXII expressed on when the dead can reach the beatific vision. The limitation on the pope's infallibility "on other matters" is frequently illustrated by Cardinal James Gibbons's recounting how the pope mistakenly called him "Jibbons".
Conditions for teachings being declared infallible
According to the teaching of the First Vatican Council and Catholic tradition, the conditions required for ex cathedra papal teaching are as follows:
the Roman Pontiff (the Pope alone or with the College of Bishops)
speaks ex cathedra, that is, when, (in the discharge of his office as shepherd and teacher of all Christians, and by virtue of his supreme apostolic authority,) he defines a doctrine
concerning faith or morals
to be held by the whole Church.
The terminology of a definitive decree usually makes clear that this last condition is fulfilled, as through a formula such as "By the authority of Our Lord Jesus Christ and of the Blessed Apostles Peter and Paul, and by Our own authority, We declare, pronounce and define the doctrine . . . to be revealed by God and as such to be firmly and immutably held by all the faithful," or through an accompanying anathema stating that anyone who deliberately dissents is outside the Catholic Church.
