Adherents.com is only a source of information about the size and geography of various faith groups. You can find out how many adherents or congregations/churches/meeting units a faith group has, how many countries the group is organized in, and where (or when) adherents are generally located.
For further information (such as doctrine, practices, history, etc.) you should consult other sources. Sometimes the original sources that our adherent statistics citations come from (many of which have web links) also present further information about the religious group. But many of these sources simply present survey or census data for a geographic area and do not have further information.
Of the many hundreds of web sites I've examined, I recommend the following for finding more information about specific faith groups:
General Directories to Web Sites about Specific Religious Groups and Denominations
The following links are primarily to web sites which focus on providing links to religious and official denominational web sites on which people describe their own faith group:
By Tsuyoshi Nakano Seminar. Great list of links to religious bodies and denominations active in contemporary Japan (Buddhist, Shinto, Christian, New Religions, etc., including groups originating outside Japanese).
Great list of links for this category, which contains many large, successful, and influential groups, but which is often neglected by more Western-centered link lists. (Part of the University of Florida's "Religion Religions Religious Studies" web site.)
This web site by University of Calgary Professor of Religious Studies Irving Hexham provides excellent academic coverage of the subject of NRMs. Some excellent resources and original material can be found here, including valuable original resources and links for African religion.
General Internet Search Engines with Link Categories for Religion and Spirituality
(These links take you directly to the religion/faith group sections in these sites. Sites are listed by order of how useful the site is for doing research on faith groups. At the bottom of this list are Search Engine sites which appear to be authorized mirrors (duplicates) of previous Search Engine sites, except for differences in advertising, home page, color schemes, etc.)
About.com -- has personable, paid, expert category hosts. Links are carefully chosen, high quality sites
Yahoo -- very extensive and well-categorized collection of links.
Dictionary.com - Great collection of links with accurate information, very wide variety of religions.
HotBot - Now has a new religion section, which is quite well organized. Has extensive links and valuable site recommendations.
Links2Go -- "Search the Web Sideways." Unusual link format, but extensive faith group categories and many good links.
Open Directory -- Calls itself the largest human-edited directory on the web. Quite a good listing of religion links.
Excite -- link lists are middle of the pack. Keyword search is pretty good.
Lycos -- category levels aren't very deep so it can be difficult to find what you want, but has a large collection.
Snap.com - Not bad. Pretty good set of faith group categories. Links are an eclectic mix of quite good and trivial.
Galaxy -- Faith group categorization is not bad. Small number of links, but they're very useful. Some technical annoyances.
AltaVista -- category levels not very deep and link collection not as balanced as most other Search sites listed here. (Extensive web-spidering yields large hit counts from keyword searches.)
Thunderstone -- What's nice is there is a large collection of links and very few graphics or ads. The organization is a mixed bag: mostly logical, but there also large numbers of un-categorized or completely misplaced links, indicating that much of the classification is automated or submissions are un-proofed.
WebCrawler -- looks like the same set of links that are at Excite.com.
LookSmart -- Same format and links as AltaVista, but with a different color scheme.
A project supported by the Lily Endowment. This archive contains data sets and research tools intended for serious statistical analysis of American religion. Data sets cover such topics as opinion surveys, religiosity measures, congregational studies, clergy attitudinal research, etc. Very useful resource for professional statisticians and researchers. Includes over 200 surveys and supporting software for generating online maps and reports for church membership.
Purchase over 20 MB of files on CD-ROM, including the statistics, addresses, etc. behind Patrick Johnstone's Operation World and David Barrett's figures. This database has not been used by Adherents.com, so it contains a large amount of information not available here. Especially strong in providing information on specific Protestant denominations in foreign countries.
1993 article by publishing findings of an in-depth academic study done by Benton Johnson (University of Oregon), Dean R. Hoge and Donald A. Luidens, funded by the Lily Endowment. Researchers interviewed 500 middle-aged, once-confirmed members of the Presbyterian Church (U.S.A.) to provide empirical evidence for various theorized reasons for the numerical decline in mainline Protestant denominations.
This project focuses on the approximately 6 to 10 million Americans who worship in religions other than Christianity and Judaism. This site has the best database you will find for Muslim mosques, Sikh gurdwaras, Hindu, Buddhist, Zoroastrian and Jain temples, and a few other type of religious centers.
Guide to various off-line books and resources within the library which provide statistics, mostly for individual denominations. (The statistical data from most of the general resources listed here can be found online in the Adherents.com database, but the historical statistical data from most of the denomination-specific resources is not available here.)
Although not purely a statistics page, this is an excellent source of membership figures for the various Catholic, Orthodox, and Apostolic (Anglican, Lutheran) churches, including dozens of non-Latin Rite Catholic churches.
Includes a wealth of population data, including a very useful listing of Population of Countries of the World and "World Population Growth from 0 to 2050". (So if you know how many adherents a religion has, but not the percentage, you can look up the total number of people in the country, or world, using these tables.)
Useful statistics tables for the whole world, such as "Population of the largest cities" [in each country], "Worlds largest cities", and "Components of population change".
Interesting but Unclassified
The following links are not among the most useful sites for doing research about religions, but they're undeniably unique and entertaining perspectives on religious statistics, classification and other topics.
Ethnologue - Languages of the World - This is an immeasurably valuable, comprehensive resource for information about the geography and statistics of languages and language speakers. (For many peoples, there is no difference between their linguistic and religious group.)