I thought this was a good short summary of modernism:
In origin at least, modernism can be regarded as a failed attempt at aggiornamento [basically, updating], its shadow side or occupational disease, aggiornamento itself being a thoroughly legitimate activity.
Most of the damage can be attributed to the kind of radical biblical scholarship, dating from the late 18th century, which cast varying degrees of doubt on the authenticity and truthfulness of the Bible. The doubts included the Resurrection. But if the Resurrection was not an historical fact, on what was Christianity based? This led to a new theory about the way God reveals himself.
For modernists there has been no revelation with an unchanging content given by God through specially appointed spokesmen and ending with the death of the last apostle St. John. In so far as God speaks to men he does so mainly or solely through personal religious experience, and what he says continually changes or is modified as the world and men’s situations change. Today this idea is called “on-going revelation”, and its interpretation by theologians “process theology”.
From this it would follow that Christian doctrine only has a symbolic significance and must continually be reinterpreted. In its present form it represents the efforts of earlier less enlightened Christians to interpret what God was saying to them through their particular experiences. Similarly with the Bible. The Bible [modernists say] is basically a record of what the Jews and early Christians felt or thought about their inner experiences, not about what was actually said or happened to them. In modernism everything takes place in the mind, rather than externally.
If revelation is through personal experience, the Christian people are the final arbiters of what is to be believed and done. The government of the Church should therefore be remodelled more along the lines of a modern popular democracy. After an exchange of experiences the people reach a consensus, which, when sufficiently widely accepted, the bishops ratify, the consensus becoming the Church’s official teaching for the time being.
The bishops are merely the people’s delegates. However a closer look at the theory reveals that it is theologians, not the people, who hold the determining position. On their own, the people are unable to articulate their experiences properly. Only with the help of theologians can they discover what God is saying to them. Theologians are the midwives of popular experience.
Although the theory has a supposedly democratic basis, it in fact turns theologians into bishops and bishops into errand boys.
Turmoil and Truth: The Historical Roots of the Modern Crisis in the Catholic Church, by Philip Trower (Ignatius Press 2003), Chapter 2.