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Reuters reported in December 2013 that a five-yearly report provided to Pope Francis by Dutch bishops visiting Rome has blamed a "drastic secularization" of society. A critical group of Dutch lay Catholics saw another important factor, the scandal of sexual abuse of minors by priests, which has afflicted many Catholic dioceses around the world, had also driven many people away, as had the closure of churches in Holland.
Wikipedia says that the proportion of Protestants in the Netherlands had fallen from 60 per cent in the early 20th century to only 16.8 per cent in 2007, saying that defections were primarily due to rising unaffiliation. This suggests that the dramatic decline in Protestant numbers is also due to "drastic secularization," although earlier and more dramatic than the fall in Catholic numbers.