Adolf Harnack 
The Constitution and Law of the Church in the First Two Centuries [PDF ebook] 

From the author’s Preface:
…The root of the organization of the Church is the proclamation of the Word of God. The Word of God took the form of a Gospel. In the Christian preaching at a very early period the Trinitarian Confession came to the front and gave the new religion its distinctive stamp. These were the strongest motive forces in the formative period of the Church. Yet we look in vain in theological literature for monographs in which their origin, their original meaning, and their development are made clear. This noticeable gap I have sought to fill, confining myself, as regards the Trinitarian Confession, to showing the motive which led at a very early period to a bipartite or tripartite formula. The result of the investigations into ‘Gospel’ will be to show that on this most important point also the Christian religion displayed from the beginning the wonderful many-sidedness, elasticity, and capacity for development which is the presupposition of its universality.

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