Although any simple answer is an oversimplification of the many reasons for doctrinal diversity within Christianity, however, I did want to add some personal beliefs as to why not all motivations underlying diverse beliefs are evil or even bad.
Ive come to the conclusion that some peripheral theological differences are almost inevitable due to our inherent differences in experiences, in knowledge and understanding. I do not think differences in opinion are completely avoidable but rather that they often are part of the moral milieu necessary for us to learn and gain moral qualities such as patience and empathy for those who are different than ourselves.
1) Regarding Changing doctrines due to changing knowledge and understanding within a single individual
Even single individuals do not believe the same as adults as they did when they were children since our own knowledge base enlarges and our understanding changes to reflect this constant change. We are, all of us, biased (dare I say tainted) to some extent by "local" cultural traditions weve inherited and been exposed to.
2) Regarding Changing doctrines due to changing life experiences.
We also become "philosophically re-directed" by certain life experiences. For example, if I am a mother of an infant who died a few hours after birth, then a prior held tradition that even infants who have not accepted Jesus go to hell; may seem completely unfair. Even attempts by others to support that doctrine by claiming God has the power to do what he wants to infants may not make damnation of the innocent infant seem just or fair to this mother. Such a doctrinal-moral dissonance may motivate and underlie the search for a better understanding or doctrine which may seem more fair.
Any new doctrinal understanding gained by such a search may be less correct OR more correct than the prior belief, and this search was motivated by a personal experience not all individuals will have and it may result in a differing belief that not all share. Still, this experience was a source of a differing doctrinal understanding of the actual justice (or unjustness) of God. Even if the new and better doctrine is more correct than the prior-held tradition one grew up with, still, it will be seen as and declared unorthodox by those holding to the prior tradition (whether correct or incorrect......).
Because of these and other reasons, I do not think we can maintain a single assembly-line level of understanding and beliefs on peripheral doctrines to the degree that we can agree on a central, core doctrine such as the claim that Jesus was in one way or another, a "redeemer.
3) There are many other reasons why beliefs in Christian doctrines we grew up with may change in a single individual.
For example, Religious Christian theists who become steeped in Judao-Christians history repeatedly experience doctrinal "shifts" as they are exposed to new historical discoveries regarding early Judao-Christian beliefs.
For example, as one becomes interested in early Judao-Christian texts such as early JudaoChristian diaries, hymns, written prayers, and other texts sacred to the earliest Judao-Christians, one will discover how beliefs in early Christianity differed from ones modern Christianity and belief. When faced with such discoveries, the discoverer is faced with a new doctrinal choice he did not imagine before the discovery.
If the discovery is a doctrinal or interpretational difference between an earlier ancient christian belief and the belief the discovering historian currently holds, he then may experience the discomfort of having to change his own belief, especially if the ancient belief is undoubtably authentic; and is superior; and leads to greater understanding; and is less doctrinally discordant than a modern belief or interpretation.
Initially, such a choice to re-align one's beliefs, is often somewhat uncomfortable. However, the experience of gaining a superior doctrine at the expense of having to give up a prior, but inferior tradition, is itself, a rewarding experience such that, when such experiences occur over and over and the experience of discovery and giving up a prior held belief for a superior and more correct belief becomes easier, then at some point the motivation towards discovery of authentic christianity and the change of opinion it forces upon the Judao-Christian historian, becomes welcomed, rather than discomforting.
However, another source of doctrinal friction may then occur when a more enlightened, but impatient and un-empathetical Christian historian-discoverer then tends to condemn another person's doctrines : Another person who is no more ignorant than the historian used to be; and is no more dependent upon mere tradition as the historian used to be. Our attitudes in discussions may cause as much argument as continued ignorance.
For example; while in this journey toward ever greater understanding of authentic early judao-christian religion, the discoverer often tends to overestimate the amount of knowledge he has achieved and becomes proud and arrogant and, though his knowledge might be superior, still, he may lack the needed christian qualities of love and charity and patience and empathy for others and become argumentative to the point that it is counterproductive and cause good but ignorant individuals to simply dig in their heels against his better information. This is not necessarily because the historian's new doctrine is inferior, but because of the antagonistic nature of many disagreements.
Clearly
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