Oath Against Modernism
The Oath against Modernism was issued by the Roman Catholic Pope, Saint Pius X, on September 1, 1910, and mandated that "all clergy, pastors, confessors, preachers, religious superiors, and professors in philosophical-theological seminaries" should swear to it.
The oath continued to be taken until July 1967 when the Congregation for the Doctrine of the Faith rescinded it. It is, however, still taken before priestly ordination by most traditionalist Roman Catholic clergy.
Previously Pius X had defined what he saw as the sin of Modernism in his encyclicals Pascendi Dominici gregis of 1907, and Lamentabili Sane.
When John Paul II issued the apostolic letter Ad Tuendam Fidem on church discipline, it provoked dissenters into claiming that the letter was a second oath against modernist thought.