You really can't go by church attendance, or other public community events for a lot of countries, since cultural and political context affects it. For instance in Russia, the largest religion by far is the Russian Orthodox Church. When compared to France, the people of Russia seem to be miniscule in their adherence to their faith. However, many Russian orthodox christians view the Church as a corrupt puppet of the Russian government, and thus a saying was born in Russia, "Corruption lives in the church, God lives in the heart." Essentially meaning that one would not have to listen to or consisently attend mass in order to believe in Christ. Another example would be Middle Eastern Christians, who oftentimes are religious but unable or unwilling to attend mass because of social tensions. This is especially true in Egypt and Iraq. There's also the fact that 75% of Middle Eastern Christians are diaspora in the West. So it's pretty difficult to narrow down who is religious and whether their belief is authentic. Any study is too subjective, and therefore not always reliable. It's probably best to just judge the result based on the number of people who call themselves Christian. Atheists will be the first to open their mouths and declare their unbelief, don't worry.