Why is that? All other belief systems have gradually built up massive "Traditions" that define the wide variety of situations that humans sometimes find themselves in, and also define the acceptable and unacceptable responses and reactions to such situations. All that Tradition structure that exists has the effect of very sharply defining the behavior and beliefs of that religion.
Interestingly, each one of these many Churches really believes that they alone truly understand the Bible and have a perfectly correct interpretation of its Teachings and its Lessons. Therefore, virtually all of them consider all of the rest to be wrong, and to some sense, heretical! I have to think that Jesus must find some dark humor in this situation! All these Churches use the very same source Manuscripts as the basis for their Faith, yet they each believe that all the others that do the same are wrong! Doesn't that seem pretty arrogant? "We're smart enough to understand the Bible and you aren't!"
To give a common example, in the period between 1840 and 1860 in America, many Protestant Denominations split in two. Some believed they saw that the Bible clearly allowed slavery, and others, from the very same Bible, were just as totally convinced that it clearly criticized slavery. In some cases, the split denominations rejoined within 50 years, but in one major case, the rejoining only occurred during the 1980s, 140 years later! And, the Southern Baptists and Northern Baptists still feel the need to have separate Churches! (But they have grown to have an assortment of other differences, too!)
Given the limitations of our human intellect, I suspect that all of our existing Churches probably have incomplete understandings of Scripture. (even including our little Church!) It wouldn't surprise me that, some day, they all discover that the "adversaries" have actually been correct all along, just as they too have been!
The various statements in the Nicene Creed were developed by the most learned Christian scholars of the time, but they were still humans! There were other scholars, then and since, who disagreed with one or more of the statements in the Nicene Creed, primarily because they interpreted the Bible to say or imply some different concepts.
Some of those groups were led by very persuasive and even charismatic leaders, and they began to develop large followings among the public. General Christians often do not recognize the subtleties and consequences of the several different understandings of the "Nature of Jesus" for example. Some people (then and still) considered Jesus to have been entirely Divine (not human in the least), while others considered Him to have been entirely human (for example, merely "adopted" by God, or several other possibilities), while still others feel that Jesus True Nature is one or another combination of the two. At Nicaea, it was decided that the "correct" understanding is that He has one specific manner of such a combination of Divine and human natures. As soon as that was agreed to, then all of the existing groups that believed any of the other understandings became "heretical" groups, groups that believed something that "wasn't Christian!"
Unfortunately, as each Church has become popular and large, they have also tended to become arrogant at believing that their answers to such matters are the only allowable answers, and that anyone who believes otherwise is therefore heretical. The extreme case of this is the (Eastern) Orthodox Church, which is so certain that they are correct that they absolutely deny the validity of all other versions of Christianity! Many Eastern Orthodox Church leaders still consider the Catholic Church and all the many Protestant Churches to all be run by Satan and therefore totally corrupt and worthless! The majority of modern Churches have less intense, but similar, attitudes toward every other Church than theirs.
Yes, we certainly think that there are some Churches that have some wrong beliefs. But we also note that even such Churches virtually always also have many "right" beliefs which they Teach to their members. People who attend such Churches wind up learning about the Lord and about Worshipping Him, and about learning how to become Devout to Him in their beliefs. Aren't those good things? Even if they are Taught in an environment where some inaccurate Teachings are also taught?
We are tempted to think that we are not in a position to be able to judge exactly how much "right" Teaching is appropriate as compared to what might be wrong, to be able to say whether a specific Church or Denomination is valid or not. We think that it is up to the Lord alone (through the Holy Spirit) to adjudge such things. The Bible states that clearly. So, when we see Churches that Teach some troublingly inaccurate aspects of Christianity, in our opinion, then it is required of us to still be supportive of those Churches, as long as they generally Teach Worship and Devotion for the Lord.
There are a number of modern Churches that we do not agree with, and which we might technically consider as somewhat heretical, but if they seem to Teach most of what is normally considered proper Christian beliefs, we do not think that we are in any position to severely criticize them!
We also have a far more complete compendium of Heresies as listed and described around 381 AD. Heresies
http://mb-soft.com/index.html
http://mb-soft.com/public/otherrel.html
http://mb-soft.com/index.html
C Johnson, Pastor,
A Christ Walk Church