Cardinal Newman, in his remarkable work An Essay on the Development of Christian Doctrine, discusses the important difference between doctrine and principle:
Principle is a better test of heresy than doctrine. Heretics are true to their principles, but change to and fro, backwards and forwards, in opinion; for very opposite doctrines may be exemplifications of the same principle. Thus the Antiochenes and other heretics sometimes were Arians, sometimes Sabellians, sometimes Nestorians, sometimes Monophysites, as if at random, from fidelity to their common principle, that there is no mystery in theology.
Newman’s explanation may appear subtle, but it helps one see past what are often distracting points of rhetoric—sometimes no more than personal moods and fads—and glimpse the heretic’s underlying attitude. This is “the solution of the paradox ‘Extremes Meet,’ and of the startling reactions which take place in individuals; viz. the presence of some one principle or condition, which is dominant in their minds from the first to the last.” This is why people at supposedly opposite extremes sound so much alike. It is this extremism, or presumption and impatience, that they have in common.
Doctrine is not only an important expression of faith, it helps protect and preserve principle. Yet for the individual, belief starts not with intellectual constructs but with deep-seated personal motives. “If I have all faith so as to move mountains but do not have charity, I am nothing” (I Cor. 13:2).