Complex sentence: After the advancement of 20th century modernism, European and American art schools embraced thinkers who rebelled against nineteenth century academic and historicist traditions, believing the "traditional" forms of art, architecture, literature, religious faith, social organization and daily life were becoming outdated.
Split into simpler sentences: After the advancement of 20th century modernism, European and American art schools embraced thinkers who rebelled against nineteenth century academic and historicist traditions. They believed the "traditional" forms of art, architecture, literature, religious faith, social organization and daily life were becoming outdated.