What is The Fastest Growing Religion?
Short answer: Christianity
Slightly longer answer: Christianity is the fastest growing religion in absolute growth, meaning the fastest growing religion in the annual increase of new adherents. The absolute growth number is calculated as the annual number of new Christian converts subtracted the number of ex-Christians + number of births subtracted number of deaths (new converts - former Christians + births - deaths).
Christianity is the fastest growing religion at both parameters, both in absolute number of new converts and absolute number of new Christians due to population growth. In all, this sums up to an additional approximately 25.2 million new Christians annually(Table J). 22.7 million due to population growth and 2.5 million due to conversions(Table J).
Another slightly longer answer: Another way to calculate growth is in relative terms. How much does a particular religion grow (or shrink) compared to last year. Since relative growth favours smaller base numbers, in terms of relative growth, the fastest growing religion in the world will be some very small obscure sect few people outside the sect have ever heard of. The mathematics of relative growth dictates that it much easier to post impressive sounding relative growth rates, if your base numbers are small. A small sect growing from 5 to 10 adherents, sums up a relative growth of 100% - but with only 5 new adherents. A slightly larger sect of 100 adherents, adding 5 new adherents to this only results in a relative growth of 5%. Relative growth favours smaller religions.
Of the 12 classic world religions, Zoroastrianism(Term 9) is the fastest growing religion. Growing at an annual pace of approximately 2.65%(Table J). But note that this is with only 58.5 thousand new adherents (zoroastrians) annually, or 0.2% of Christianity's approx. 25 millions new Christians.
The 12 classic world religions are(Term 11): Christianity, Islam, Hinduism, Buddhism, Sikhism, Judaism, Bahá'í, Confucianism, Jainism, Shinto, Taoism and Zoroastrianis
What about Islam. I've been told/heard/read that Islam is the Fastest Growing Religion: Islam is the second fastest growing religion. Tagging Christianity by an annual growth of approximately 2.6 million less Muslims than Christians. Approx. 1 million less due to slower population growth and approx. 1.5 million less due to fewer conversions to Islam than to Christianity(Table J).
Longer answer:
Christianity is the fastest growing religion and projections of current trends will must likely result in it progressively distancing it further from other large religions.
Christianity is the fastest growing religion at the only parameter for which such a statement has any meaning: the absolute number of new adherents. Other religions are showing faster growth trends at relative growth, but this is a construed and almost meaningless way to calculate growth rates, since it speaks nothing of how many new adherents the various religions in actual get.
Religions that obtain the largest part of their overall growth through basic population increase (rather than conversions) are going to be the most vulnerable to population growth stagnation. Of the four largest religions this will be most vivid for (in order of affect) Hinduism, Islam and Buddhism, which all source a much larger part of their growth from population compared to Christianity(Table M).
All the larger religions are still growing at a fast pace through basic population increase. A rising sea floats all boats. But of a greater future importance will be the comparatively large number of people who chose to become Christians through conversions. At an annually rate of approx. 2.5 million, the number of people who freely chose to become Christians is more than twice the combined number of people converting to all the rest of the twelve tabulated world religions(Table L).
As a result of the demographic transition phase from a high fertility / high mortality to low fertility / low mortality, the demographic trend the last half a century has been steadily towards lower population growth rates. Consequentially with a continuation of the current falling population growth rates, conversions are destined to become increasingly more important for the overall growth.
The world average total fertility rate has declined from 3.50 child per woman in 1989 to 2.61 child per woman in 2008(Table N). A 25% decline, resulting in an average 0.89 children less per woman. In just twenty years. From a level clearly above the replacement level, to a number only slightly over or around the replacement level. The replacement level is the point at which the number of births per woman will over time result in a stable population; neither rising now shrinking, but exactly replacing itself. This declining fertility is shared by all the larger religions. But not shared equally. Starting from 1989 the weighted average fertility rates for all Christian dominated countries has declined from an average of 3.26 children per woman to 2.60 children per woman(Table N). A 20% decline or an average of 0.66 less children per woman. From a level below the world average to a level about on par with the world average. Whereas the weighted average fertility rates for all Muslim dominated countries has declined from an average of 5.17 children per woman to 3.30 children per woman. A 36% decline or an average 1.87 less children per woman. Starting from a higher level the Muslim percentwise decline is only about a third higher than the Christian (36% vers. 25%), but in absolute numbers this translates into about three times as large a drop in fertility (1.87 vers. 0.66).
This relative smaller declined for Christian predominated countries is mostly due that many Christian countries have already passed through the demographic transition phase, and starting from a lower fertility level (in 1989) not declining so fast - indeed the last two years have showed an increase. But also due that a number of Christian countries (in south Sahara Africa) are still early in their transition phase and so have a very high fertility.
The average fertility rate for Muslim predominated countries in 2008 remains higher than for Christian predominated countries, although the gap from 1989 has been reduced by an approximately 44%. By a projecting of the current trends - slow Christian fertility declined - rapid Muslim decline, the average Muslim and world/Christian fertility rates will converge in around 2022 or 2023. Overall population growth can, and probably will, still be higher for Muslim predominately countries for a number of years thereafter. But this will be due to a demographics momentum and the consequence of an aging population, which will even out later in the century.
Another element to factor in is an approximation of the replacement level. A often used rule of thumb puts this at around 2.1 children per women. This indicates that a very high rate of newborn babies will live to the end of their reproductive age (set at around 45 years). However this approximation will only fit Western and industrialised countries with a highly developed health infrastructure, a general satisfactory nutritional level, an educated population, a well functioning sanitary system, etc. which all contribute to lower mortally rates and lengthen life span. Countries with lower standards will sadly invariable experience a higher mortality and thus a higher replacement level. Sometimes considerable higher. Someplace as high as 3.5 children per women. Indicating a comparatively large number of newborn babies will not live to the end of their reproductive age. Populations with a high replacement level will likewise need a high total fertility for the population to remain stable. Christian predominantly nations are on average better off economically, and in regard to health and education, meaning they will have a lower replacement level, than the average for the rest of the world, and will need a lower fertility rate for the same population growth.