It started when Catholics became more educated about the Church’s interpretation of the Bible based on scholarly understanding which has evolved since the 1940s when a papal encyclical encouraged scholars to use the historical critical method to study scripture.
I think calling the Bible “a myth” is not accurate, but also reading the Bible literally is not accurate either. When interpreting scripture, you have to take into account the context in which the writer was writing and the typical style of literature of the times. Unlike Muslims and some evangelical Christians, Catholics believe that God did not tell the scripture writers what to write, he instead inspired them to write about God and teach people about his covenant with humankind and he let the writers use the talents he gave them to convey those messages.
Unlike our on times where we are concerned with details and accuracy, the biblical writers were more concerned about the theological accuracy than the historical accuracy. They wrote about the events that had occurred in a manner that conveyed God’s message to the people. For example, did Moses exist? I think most scholars would say yes. Did the events recorded in Exodus happen exactly as recorded, probably not, but that’s not important, what is important is understanding the covenant that God made with his people and how the people would lapse into unfaithfulness at times, but God continued to remain faithful to the covenant.
You question about whether the resurrection actually happened? I don’t think any Catholic scholar would say no. The resurrection is one of the few stories which is told in all 4 gospels, this is strong evidence that it actually occurred, being that the gospel accounts come from different communities and oral traditions. Yet all 4 accounts of the resurrection are different. This tells us that the resurrection did occur, but the apostles witnessed something which they did not fully understand at the time, and their experience of it was different, thus different accounts evolved over time in different communities. But despite the differences in detail, all 4 stories convey the important theological message, that Jesus was not dead, but instead God resurrected him as promised and that he is our saviour, even though he did not fit the mold most people thought the saviour would take (ie a king who conqueres land and frees Israel).
So the Biblical authors may have used allegories and stories to convey a theological truth, but that does not make that truth any less of a truth, it just represents a different way of expressing that truth than we are accostomed to using in our modern world.