Thought it still only dates back to the mid-nineteenth century, I have exactly this problem with Marxism. On the one hand, there are certain ideas that are apparently fixed within the doctrine, namely "dialectical and historical materialism", as methods for understanding and interpreting nature and society. On the other hand, a great deal has happened since the days of Marx and Lenin and there is a clear need to revise some of it's ideas so they correspond to current levels of knowledge and conditions.
This is especially true given that Marxism didn't necessarily produce the paradise it originally intended and capitalism seems to have won. Imagine if you are Christian and the Pagans come back until you are hopelessly outnumbered and you'll get the idea. You are left asking questions such as: "Was our beliefs false?", "Does it have to be reinvented?", "Or should we stick to the true principles in the belief that they will ultimately be proven correct in the future?"
There is a continuous battle and tension between these two competing demands, of being ideologically orthodox and internally consistent with the doctrine, with responding to new evidence and experience as it comes to light. There is therefore a need to "change the teaching" if you will but there is a limit before it ceases to be Marxism and becomes something "else" that is not-Marxism, i.e. heretical views that are "revisionist".
Living with these ideas is difficult, firstly because they are so unpopular, second because information to understand them "correctly" in the "approved" manner is generally not widely available in English (but in Russian or Chinese) and third, many of the issues that come up in life and contemporary politics are genuinely new and require original thinking starting from Marxism's first principles of dialectical materialism.
For example, Communist Parties are having internal battles over trans-rights because materialism implies gender is objectively real, whilst dialectics would suggest it is not fixed but can be fluid and capable of change. Depending on which side you emphasise, you get different and contradicting opinions.
Naturally, Communists have proceeded to purge each other for their respective "heresies" and can't agree on anything and mutually accuse each other of being deviant and heretical or "revisionist". I suspect this problem will be very familiar to the religious believers and is probably not unlike Christians and Muslims having the same problem reconciling the new demands posed by society with their pre-existing scripture and theology. The only difference for us Marxists is it's more to do with politics and philosophy than theology and religion.
