The practical aim of all true religion is to bring each individual soul under the immediate guidance of the divine Spirit. The divine Spirit communicates himself to the soul by means of the sacraments of the church. The divine Spirit acts as the interpreter and criterion of revealed truth by the authority of the church. The divine Spirit acts as the principle of regeneration and sanctification in each Christian soul. The same Spirit clothes with suitable ceremonies and words the truths of religion and the interior life of the soul in the liturgy and devotions of the church. The divine Spirit acts as the safeguard of the life of the soul and of the household of God in the discipline of the church. The divine Spirit established the church as the practical and perfect means of bringing all souls under his own immediate guidance and into complete union with God. This is the realization of the aim of all true religion. Thus all religions, viewed in the aspect of a divine life, find their common centre in the Catholic Church. The greater part of the intellectual errors of the age arise from a lack of knowledge of the essential relations of the light of faith with the light of reason; of the connection between the mysteries and truths of divine revelation and those discovered and attainable by human reason; of the action of divine grace and the action of the human will. The early Greek and Latin fathers of the church largely cultivated this field. The scholastics greatly increased the riches received from their predecessors. And had not the attention of the church been turned aside from its course by the errors of the XVIth century, the demonstration of Christianity on its intrinsic side would ere this have received its finishing strokes. The time has come to take up this work, continue it where it was interrupted, and bring it to completion. Thanks to the Encyclicals of Pius IX. and the decisions of the Vatican Council, this task will not now be so difficult. Many, if not most, of the distinguished apologists of Christianity, theologians, philosophers, and preachers, either by their writings or eloquence, have already entered upon this path. The recently-published volumes, and those issuing day by day from the press, in exposition, or defence, or apology of Christianity, are engaged in this work.