August the 23rd of this year marks the centenary of the decree of the Sacred Congregation of Rites by which Pius IX instituted the feast of the Sacred Heart as a common feast for the whole Church. His successor Pius XII takes this opportunity to write his latest encyclical letter in order to explain and defend what has become such a popular devotion of the faithful during the past century. But it is a devotion which has had a much longer history than that, as the Pope is at pains to prove; indeed he here maintains that fundamentally it is an ancient and traditional devotion of the faithful, the only thing new about it being the new form it has taken on during the past three centuries owing to the influence of certain holy men and women. It is this new form which gives reason for the apologetic or defensive character of the encyclical.