Marches, teach-ins, petitions, boycotts, grass-roots collectives: radical initiatives that readily conjure up images of the May 1968 protests in Paris or the student sit-ins at the London School of Economics. Yet, in the summer of 1968, the publication of a papal encyclical was to provoke similar forms of protest.

This unlikely affront to the public conscience was, of course, Pope Paul VI’s encyclical letter condemning the use of artificial contraception, Humanae Vitae, which was published on 29 July.