Why and how did Europeans openly challenge the Catholic Church, church doctrine, and the bible?

By the time of the Reformation, European society was experiencing greater social mobility. The rising middle class was challenging the established social order. Humanists stressed the central importance of individual talents and performance regardless of noble descent. Printing made books widely available and facilitated access to the Bible for literate lay people. People were increasingly looking for a personalized approach to religion to accommodate diverse spiritual needs and guide individuals in their quest for salvation.
Meanwhile the Catholic Church as an institution remained deeply committed to medieval ritualism. To satisfy its financial needs it relied on payment for various religious ceremonies as well as on the sale of indulgences, which promised their buyers forgiveness of sins if they repented sincerely in addition to making their purchase.
Martin Luther, a pious German Augustinian monk and a professor of theology who was of modest middle class origin, resented the indulgences, which he found incompatible with a personal connection to God. He posted his objections. Because he was a forceful and effective writer in both German and Latin, he soon became very popular with the German reading public. When the Papacy denounced him as a dangerous heretic, he responded by denying the notion of clergy as a separate estate and the Church’s claims to be the sole mediator between people and God. He stressed the importance of the individual believer's conscience and faith. He also translated the Bible into German and argued that Scripture as the Word of God was much more reliable than Church traditions. His numerous followers formed their own religious communities, creating Protestantism as an independent religion.