At the time, we felt so infallible in our rightness we grabbed the proverbial bull by the horns exposing ourselves to a wealth of trouble.
You're not infallible like me.
The first decree (Decretum de fide et ecclesia) declared that the Catholic Church has no right to introduce new dogmas, but only to preserve in its original purity the faith once delivered by Christ to His apostles, and is infallible only so far as it conforms to Holy Scripture and true tradition; the Church, moreover is a purely spiritual body and has no authority in things secular.
The word has thus the general sense of "certainty"; we may, e.g., speak of a drug as an infallible specific, or of a man's judgment as infallible.
In modern Protestantism, on the other hand, the idea of an infallible authority whether in the Church or the Bible has tended to disappear, religious truths being conceived as valuable only as they are apprehended and made real to the individual mind and soul by the grace of God, not by reason of any submission to an external authority.