In the late 1940s Leonard Feeney, S. J. began to teach that there is no salvation outside the Church. He was correct in saying that there were official teachings, even definitions, on that score. But his tragic error came when he adopted Protestant method, thinking that in that way he would be one of the only true Catholics! We spoke of his protestant method with good reason. First, he was excommunicated for disobedience, refusing to go to Rome to explain his position. Then the Holy Office, under Pius XII, sent a letter to the Archbishop of Boston, condemning Feeney's error. (It is known that Pius XII personally checked the English text of that letter). In the very first paragraph pointed out what is obvious: we must avoid private interpretation of Scripture -- for that is strictly Protestant. But then the letter said we must also avoid private interpretation of the official texts of the Church. To insist on our own private interpretation, especially when the Church contradicts that, is pure Protestant attitude. What the disobedient Feeney said amounted to this: he insisted that all who did not formally enter the Church would go to hell. Hence he had to say, and he did say, that unbaptized babies go to hell. Further, all adults who did not formally enter the Church - get their names on a parish register - would also go to hell, even if they never had a chance to hear there was a Church, e.g., those in the western hemisphere during the long centuries before Columbus. Therefore Feeney consigned literally millions upon millions to hell, even though He gave them no chance. Not just the documents of the Church as interpreted by the Church should have kept him from this: merely common sense, and the realization that God is not only not a monster, but is infinitely good - that alone should have stopped him. We have, then, most ample reason for calling his error tragic. Even the sexually immoral do not deny that God is good. Feeney does worse than they. I regard to the damnation of infants, tragically, Feeney cited a text of Pius IX (quoted below) saying that no one goes to hell without grave voluntary sin - babies of course have no voluntary sin. Feeney actually ridiculed the text of Pius IX and charged Pius IX with the heresy of Pelagianism, saying (in Thomas M. Sennott, They Fought the Good Fight, Catholic Treasures, Monrovia CA. 1987, pp. 305-06): "To say that God would never permit anyone to be punished eternally unless he had incurred the guilt of voluntary sin is nothing short of Pelagianism... . If God cannot punish eternally a human being who has not incurred the guilt of voluntary sin, how then, for example can He punish eternally babies who die unbaptized?"
Link (here) to read the full article by Fr. William Most entitled
The Tragic Errors of Fr. Feeney