Here are some figures taken from D. Barrett's World Christian
Encyclopedia, published by Oxford University Press in 1982. I will set
them out in text rather than tabular form, in case the format does not
survive Web downloading.

In 1900, 26.9% of the world's population was Western Christian, while
Islam accounted for 12.4%. In 1980 the figures were 30% and 16.5%
respectively. The projection for 2000 is 29.9% and 19.2%. Percentages for
other religions are fairly static, and since 1970 the total of atheists
has, surprisingly perhaps, experienced a slow decline.

These figures are of considerable significance. Over the course of this
century, the absolute proportion of Muslims in the world has jumped by a
quite staggering amount. This has come about partly through conversion,
but more significantly through natural increase. And the demographic bulge
in the modern Muslim world means that this growth will continue.