Unitatis Redintegratio §10

¹⁰ Sacred theology and other branches of knowledge, especially of a historical nature, must be taught with due regard for the ecumenical point of view, so that they may correspond more exactly with the facts. It is most important that future shepherds and priests should have mastered a theology that has been carefully worked out in this way and not polemically, especially with regard to those aspects which concern the relations of separated brethren with the Catholic Church. This importance is the greater because the instruction and spiritual formation of the faithful and of religious depends so largely on the formation which their priests have received. Moreover, Catholics engaged in missionary work in the same territories as other Christians ought to know, particularly in these times, the problems and the benefits in their apostolate which derive from the ecumenical movement.