So numbers on their own prove neither truth nor righteousness.
But the main point is this.
In Islamic countries, in the last 1,300 years, there has been a essential Islamic tradition which has disallowed all criticism of Islam, the Koran and Mohammed.
What we also have, then, is the lineal progeny of Islam through generations and generations of Muslim families. Millions of Muslim families that never considered, even for one moment, even the very possibility of not being Muslim or changing their faith.
Now tell me that one billion Muslims tells us that much about either the truth or the righteousness of Islam.