Strictly Protestant

    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