THE CHURCH CANNOT ADAPT OR MODIFY HER DOCTRINE

“The underlying principle of these new opinions is that, in order to more easily attract those who differ from her, the Church should shape her teachings more in accord with the spirit of the age and relax some of her ancient severity and make some concessions to new opinions. Many think that these concessions should be made not only in regard to ways of living, but even in regard to doctrines which belong to the ‘deposit of the faith’. They contend that it would be opportune, in order to gain those who differ from us, to omit certain points of her teaching which are of lesser importance, and to tone down the meaning which the Church has always attached to them. […]  Such a policy would tend rather to separate Catholics from the Church than to bring in those who differ. There is nothing closer to our heart than to have those who are separated from the fold of Christ return to it, but in no other way than the way pointed out by Christ. […] History proves clearly that the Apostolic See, to which has been entrusted the mission not only of teaching but of governing the whole Church, has continued "in one and the same doctrine, one and the same sense, and one and the same judgment," -- Const. de fide, Chapter iv. -- In this matter the Church must be the judge, not private men who are often deceived by the appearance of right."

(Leo XIII. Letter Testem Benevolentiae to Card. James Gibbons, January 22, 1899).