Note that mid and south America were almost exclusively Catholic a century ago, the percentage of the Christian population being genuinely Christian is likely to have increased as well.
The danger with statistics is how many are really born again and saved Christians compared to nominal Christians who live like the devil...even the mafia would be included as believers
Because of population growth there would certainly be more true believers today than ever before.
Quest (10-09-2015)
Thankfully the mafia don't affect statistics much except perhaps in some small town where they make up a large chunk of the population. Maybe we could find some town where there are 100% nominals and 0% true believers. People lived like the devil in the West in 1910 too but it was typically less upfront. That's not that easy to compare at the statistical level. Statistically, there are far more people who are Christians in name only in the West and especially in Europe than 100 years ago, irrespective of how they live. So those areas represent a definite decline even though the chart would indicate a status quo.