Since Canada has been a nation, it has been religiously dominated by Christianity, particularly Roman Catholicism.
Eastern Canada experienced an influx of Catholic missionaries early in the nation's history and the presence of French culture further embedded Catholicism in the country. As the population migrated west, Catholicism went as well.
As in America, the unaffiliated are increasing in Canada.
Islam, although still very much in the minority as a religion in Canada, has experienced growth in recent years.