I do not know how “credible” the CIA is but they report 10% - 9% Coptic and 1% other. Where do you get your figure of 30%? I would appreciate knowing.
https://www.cia.gov/library/publications/the-world-factbook/geos/eg.html
CIA demographic figures are notoriously PC. They infact go out of their way to report official government figures.
A great example is Nigeria, where all independent sampling statuses have shown the Christians to be between 55-60% of the population but the CIA continues to use the old 40-45% number to avoid offending Nigeria
Malaysia is another great example. Muslims in Malaysia are under 45% (with the Indians, Chinese and Christians forming a majority) but the CIA continues to show Muslims as 50%+
Please keep in mind that this is the PUBLIC site of the CIA and can be read by anyone. As such this site is a DIPLOMATIC statement as opposed to a true research site. Anything written on that site is considered an official policy of the United States. Hence the PC nature of the site.
You can rest assured that the internal CIA analyses are far more accurate
Regarding the Coptic numbers, the most credible sources are either independent agencies that have done sampling censuses (as a true census is impossible) or teh Coptic church itself.
Keep in mind that hte Coptic church actually LOWERS its numbers to keep out of trouble from the Invaders. The last coptic church estimate was LOWERED to 14-20 million (true estimates are 20-25 million).
Even at 20 million that would form 25% of the Egyptian population, which is far higher than the 10% official numbers.