How come the Pope is supposed to be infallible?
Well, look first at what "infallible" is supposed to mean. The First Vatican Council in 1870 said the following: "We teach and define as a divinely revealed dogma that when the Roman pontiff speaks EX CATHEDRA, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals. Therefore, such definitions of the Roman pontiff are of themselves, and not by the consent of the church, irreformable."
To me, the logic is clear enough. The
Church has to have a certain authority necessary for her to preserve the Apostolic faith. The Pope has to be able to exercise that authority under certain
conditions to prevent her falling into heresy. Only in such extreme and rare circumstances does Christ's promise to the Church (that the gates of hell will
not prevail against her) guarantee that the Pope will be preserved from error. But being infallible, in that very limited sense, does not mean being unable to
make a mistake, let alone being unable to sin. There have been lots of evil popes, and lots of popes who made mistakes of judgement or siad things that were
untrue - it is just that none of them, however bad they were, ever formally taught EX CATHEDRA "a doctrine
concerning faith or morals to be held by the whole church" that wasn't true. John Henry Newman looked into this quite closely...
How John Henry Newman explained it (taken from his "Letter to the Duke of Norfolk"):
'[Neither] Pope nor Council are on a level with the Apostles. To the Apostles the whole revelation was given, by the Church it is transmitted; no simply new truth has been given to us since St. John's death; the one office of the Church is to guard "that noble deposit" of truth, as St. Paul speaks to Timothy, which the Apostles bequeathed to her, in its fulness and integrity. Hence the infallibility of the Apostles was of a far more positive and wide character than that needed by and granted to the Church. We call it, in the case of the Apostles, inspiration; in the case of the Church, assistentia.
Of course there is a sense of the word "inspiration" in which it is common to all members of the Church, and therefore especially to its Bishops, and still more directly to those rulers, when solemnly called together in Council, after much prayer throughout Christendom, and in a frame of mind especially serious and earnest by reason of the work they have in hand. The Paraclete certainly is ever with them, and more effectively in a Council, as being "in Spiritu Sancto congregata;" but I speak of the special and promised aid necessary for their fidelity to Apostolic teaching; and, in order to secure this fidelity, no inward gift of infallibility is needed, such as the Apostles had, no direct suggestion of divine truth, but simply an external guardianship, keeping them off from error (as a man's good Angel, without at all enabling him to walk, might, on a night journey, keep him from pitfalls in his way), a guardianship, saving them, as far as their ultimate decisions are concerned, from the effects of their inherent infirmities, from any chance of extravagance, of confusion of thought, of collision with former decisions or with Scripture, which in seasons of excitement might reasonably be feared.
… But since the process of defining truth is human, it is open to the chance of error; what Providence has guaranteed is only this, that there should be no error in the final step, in the resulting definition or dogma.
Accordingly, all that a Council, and all that the Pope, is infallible in, is the direct answer to the special question which he happens to be considering; his prerogative does not extend beyond a power, when in his Cathedra, of giving that very answer truly. "Nothing," says Perrone, "but the objects of dogmatic definitions of Councils are immutable, for in these are Councils infallible, not in their reasons," &c.
… And in like manner, as regards the precepts concerning moral duties, it is not in every such precept that the Pope is infallible. As a definition of faith must be drawn from the Apostolic depositum of doctrine, in order that it may be considered an exercise of infallibility, whether in the Pope or a Council, so too a precept of morals, if it is to be accepted as from an infallible voice, must be drawn from the Moral law, that primary revelation to us from God.
That is, in the first place, it must relate to things in themselves good or evil. If the Pope prescribed lying or revenge, his command would simply go for nothing, as if he had not issued it, because he has no power over the Moral Law. If he forbade his flock to eat any but vegetable food, or to dress in a particular fashion (questions of decency and modesty not coming into the question), he would also be going beyond the province of faith, because such a rule does not relate to a matter in itself good or bad. But if he gave a precept all over the world for the adoption of lotteries instead of tithes or offerings, certainly it would be very hard to prove that he was contradicting the Moral Law, or ruling a practice to be in itself good which was in itself evil; and there are few persons but would allow that it is at least doubtful whether lotteries are abstractedly evil, and in a doubtful matter the Pope is to be believed and obeyed.
However, there are other conditions besides this, necessary for the exercise of Papal infallibility, in moral subjects:-for instance, his definition must relate to things necessary for salvation. No one would so speak of lotteries, nor of a particular dress, nor of a particular kind of food;-such precepts, then, did he make them, would be simply external to the range of his prerogative.
… The infallibility, whether of the Church or of the Pope, acts principally or solely in two channels, in direct statements of truth, and in the condemnation of error. The former takes the shape of doctrinal definitions, the latter stigmatizes propositions as heretical, next to heresy, erroneous, and the like. In each case the Church, as guided by her Divine Master, has made provision for weighing as lightly as possible on the faith and conscience of her children.'
By the way, the Pope recently outlined his priorities (March 2009). From http://chiesa.espresso.repubblica.it/articolo/1337492?eng=y:
The first priority for the Successor of Peter was laid down by the Lord in the Upper Room in the clearest of terms: "You… strengthen your brothers" (Lk 22:32). Peter himself formulated this priority anew in his first Letter: "Always be prepared to make a defence to anyone who calls you to account for the hope that is in you" (1 Pet 3:15). In our days, when in vast areas of the world the faith is in danger of dying out like a flame which no longer has fuel, the overriding priority is to make God present in this world and to show men and women the way to God. Not just any god, but the God who spoke on Sinai; to that God whose face we recognize in a love which presses "to the end" (cf. Jn 13:1) - in Jesus Christ, crucified and risen. The real problem at this moment of our history is that God is disappearing from the human horizon, and, with the dimming of the light which comes from God, humanity is losing its bearings, with increasingly evident destructive effects.
Leading men and women to God, to the God who speaks in the Bible: this is the supreme and fundamental priority of the Church and of the Successor of Peter at the present time. A logical consequence of this is that we must have at heart the unity of all believers. Their disunity, their disagreement among themselves, calls into question the credibility of their talk of God. Hence the effort to promote a common witness by Christians to their faith - ecumenism - is part of the supreme priority. Added to this is the need for all those who believe in God to join in seeking peace, to attempt to draw closer to one another, and to journey together, even with their differing images of God, towards the source of Light - this is interreligious dialogue. Whoever proclaims that God is Love "to the end" has to bear witness to love: in loving devotion to the suffering, in the rejection of hatred and enmity - this is the social dimension of the Christian faith, of which I spoke in the Encyclical Deus Caritas Est.