The comparison of the two Councils is caused by the observation of the fact that the Second Vatican Counci1 has widely been regarded as arevision of the Counci1 of Trent. It is a reality that Vaticanum II, conforming to the objective given to it by Pope John XXIII, avoided condemning divergent views in its doctrinal decrees. The Tridentinum had very strict1y marked the doctrinal differences separating Protestantism from Catholicism - and it had been obliged to do so - in order to overcome the uncertainty that had arisen in matters of faith. In the decree that deals with oecumenism Vaticanum II works out what is common property of faith. In the Consti­ tution on Divine Revelation it deepens the conception of tradition which had been defined by the Tridentinum. In the Constitution on the Church it fiHs in a manifest gap in the Tridentine doctrinal decrees by producing a comprehensive ecclesiologia, supplementing Vaticanum I, too. Its state­ ments are different, but not contrary, as the substance of Christian faith has again and again to be looked at, to be thought over, to be deepened anew. The fact that the two Councils had been embedded in the respective historical situations becomes still more evident from the reform decrees. At Trent the latter were understood to constitute a "reform" i. e. a re­ establishment of the original form of the Church; Vaticanum II was set the task of the "Aggiomamento" by Pope John XXIII.