2 Since many, however, of those who profess to believe in Christ differ from each other, not only in small and trifling matters, but also on subjects of the highest importance, as, e.g., regarding God, or the Lord Jesus Christ, or the Holy Spirit; and not only regarding these, but also regarding others which are created existences, viz., the powers and the holy virtues; it seems on that account necessary first of all to fix a definite limit and to lay down an unmistakable rule regarding each one of these, and then to pass to the investigation of other points.
For as we ceased to seek for truth (notwithstanding the professions of many among Greeks and Barbarians to make it known) among all who claimed it for erroneous opinions, after we had come to believe that Christ was the Son of God, and were persuaded that we must learn it from Himself; so, seeing there are many who think they hold the opinions of Christ, and yet some of these think differently from their predecessors, yet as the teaching of the Church, transmitted in orderly succession from the apostles, and remaining in the Churches to the present day, is still preserved, that alone is to be accepted as truth which differs in no respect from ecclesiastical and tradition.
Source: De Principiis (New Advent)