Complex sentence: This brotherly love was the distinguishing mark of Christ's Church from the very beginning; the first Christian communities and the whole Church of the early centuries down to the time of the catacombs was one grand purgatorial society.
Split into simpler sentences: This brotherly love was the distinguishing mark of Christ's Church from the very beginning. The first Christian communities and the whole Church of the early centuries down to the time of the catacombs was one grand purgatorial society.