Growth is not one thing
Population totals can rise because of births, migration, changes in identification or improved counting. A reported annual rate may not measure all of these factors in the same way.
Comparative religious demography
How fast are Christianity and Islam growing?
Historical sources have offered very different estimates about the size and future direction of Christianity and Islam. This page preserves those comparisons while explaining why long-range forecasts should be read cautiously.
Read the evidence carefully
What the older estimates show
The figures below come from the historical material preserved in this essay. They are not a current census and should not be treated as precise predictions for 2026 or beyond.
Different methodsSources counted affiliation, identity, population share or projected change in different ways.
Uneven datesThe comparisons span the 1970s through forecasts reaching into the 2020s and beyond.
Uncertain forecastsBirth rates, conversion, disaffiliation, migration and definitions all affect the result.
A note on interpretation
A higher projected growth rate does not automatically mean that one tradition will soon become dominant. Percentages may describe different populations, time periods or measurement systems. The safest conclusion is that the sources disagree and that demographic forecasts change as their assumptions change.
Archived comparison
Estimates for Christianity
These historical entries are retained as an account of how researchers and commentators presented the question at the time.
| Source | Period | Reported number or share | Reported growth |
|---|---|---|---|
| Readers Digest | 1985 | 1.0 billion | — |
| Al-Islam, citing an unspecified source | 1994–1995 | — | 1.46% |
| U.S. Center for World Mission | 1970–1996 | 33.7% to 33.9% | 2.3% |
| John Gary | 2025 forecast | 3 billion | — |
| UK Christian Handbook | 2000–2010 | 27.7% to 27.1% | — |
| Barrett and Johnson | 2000–2025 | 2.015 to 2.711 billion | — |
| Todd Johnson | 2000–2025 | 2.090 to 3.023 billion; 33.4% to 35.5% | — |
| World Christian Encyclopedia | 2000 | 2.0 billion; 37.9% | — |
Archived comparison
Estimates for Islam
The historical table records projections and growth claims made using sources that differed in scope, date and methodology.
| Source | Period | Reported number or share | Reported growth |
|---|---|---|---|
| Readers Digest | 1985 | 700 million | — |
| Al-Islam, citing an unspecified source | 1994–1995 | — | 6.4% |
| U.S. Center for World Mission | 1970–1996 | — | 2.9% |
| John Gary | 2025 forecast | 2 billion | — |
| Barrett and Johnson | 2000–2025 | 1.22 to 1.89 billion | — |
| Todd Johnson | 2000–2025 | 1.16 to 1.72 billion; 18.5% to 20.2% | — |
| Samuel Huntington | 2025 forecast | 30% | — |
What can be concluded?
Three careful takeaways
Forecasts are conditional
Predictions reaching toward 2025 or 2200 depended on assumptions about fertility, conversion and affiliation. When assumptions change, the forecast changes too.
Definitions matter
Christian and Muslim populations are internally diverse. Counts may include cultural affiliation, active practice, formal membership or a broad self-description.
Comparison is not competition
Demographic size does not determine the value of a tradition or the sincerity of its adherents. Numbers provide context, not a ranking of beliefs.
Continue reading
Related material
Use these existing pages to place the historical comparison alongside broader introductions and comparative essays.
Comparison of Christianity and IslamCompare central beliefs, practices and histories.
ChristianityBegin with the site's wider Christianity collection.
IslamExplore the site's introductory material about Islam.
Religions of the worldPlace both traditions within a broader survey.
Compare beliefsRead more about comparing religious worldviews responsibly.
Predictions about the futureReview related historical forecasts.
Reader guide
Frequently asked questions
Are the figures on this page current population counts?
No. They are historical estimates and forecasts preserved from the source essay. They should not be presented as a current demographic dataset.
Why do sources give different totals?
Researchers may use different definitions of affiliation, geographic coverage, dates, surveys and projection assumptions.
Does a growth-rate comparison predict which religion will become dominant?
No. A rate is meaningful only with its time period, baseline and method. Forecasts can also be altered by fertility, migration, conversion and disaffiliation.
Why are some old forecasts included?
Their dates and assumptions are part of the historical record of public discussion about religious demography. They are labelled as forecasts rather than facts.
Where can I find broader background?
Start with the basic religious information collection or the glossary for unfamiliar terms.