The growth of Islam has a lot to do with the birth rate in Muslim countries compared to most of the rest of the world. Check out the number of children per woman of child-bearing age in those countries versus Europe, for example. As you point out, the percentage of people with no religion may be decreasing because of the number of Muslim women who are having large numbers of children.
Communism really does not have that much to do with European atheism. Please see this article in Wikipedia: By the way, you will note that Canada is cited among the countries with higher rates of atheism. It makes a poor Canuck proud, (says Valteron wiping away a patriotic tear).
"Demographics
Main article: Demographics of atheism
The percentage of people in European countries who said in 2005 that they “believe there is a God”.It is difficult to quantify the number of atheists in the world. Different people interpret “atheist” and related terms differently, and it can be hard to draw boundaries between atheism, non-religious beliefs, and non-theistic religious and spiritual beliefs. Furthermore, atheists may not report themselves as such, to prevent suffering from social stigma, discrimination, and persecution in certain regions.
Despite these problems, atheism is known to be relatively common in Western Europe, Australia, New Zealand, Canada, former and present Communist states, and to a lesser extent, the United States. A 1995 survey attributed to the Encyclopædia Britannica indicates that the non-religious make up about 14.7% of the world’s population, and atheists around 3.8%.[75]
According to a study by Paul Bell, published in the Mensa Magazine in 2002, there is an inverse correlation between religiosity and intelligence. Analyzing 43 studies carried out since 1927, Bell finds that all but four reported such a connection, and concludes that “the higher one’s intelligence or education level, the less one is likely to be religious or hold ‘beliefs’ of any kind.” A survey published in Nature confirms that belief in a personal god or afterlife is at an all time low among the members of the National Academy of Science, only 7.0% of which believed in a personal god as compared to more than 85% of the US general population.[76]
A recent poll (November-December 2006) published in the Financial Times gives more recent rates for the USA and five European countries; this poll shows that Americans are more likely than Europeans to believe in any form of God or Supreme Being (73%). Of the European adults surveyed, Italians are the most likely to express this belief (62%) and, in contrast, the French are the least likely (27%). Indeed, in France, the rate of people who declares to be atheist rises to 32% (and 32 % of agnostics).[77]"