The only ways that any country gets 99.8% of it's citizens following one religion is either by falsifying data (e.g. threatening anyone who denies that religion so that they pretend adherence) or by persecuting those of other religions (or none) to the point where they flee the country. I recently visited Armenia and met a very genial restaurateur who fled Iran because he is a Christian. In Saudi Arabia, to say Mass is illegal. IMO, countries such as these are yet to emerge from the Middle Ages.
This is a great list @Slkrr9. Cannot thank you enough for sharing this awareness on global Islamic statistics. However IMO it is slightly obsolete (based on data from Association of Religion Data Archives for the year 2010.) which did not include Kosovo, the declaration of Independence of which was not made official yet by the UN until much later. Perhaps more recent may give light to a more accurate reading such as from the 2017 Pew Research Centre or the more recent statistics from The World Factbook by the CIA, the reports of which may be considered unbiased, neutral and might perhaps dispel perceptions of data falsification purported by @CardinalSin. God knows best.