Many non-believers say Christianity is the most religion in the world, is that true?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We do not know the percentage of the Christians in the world and we do not know whether or not they are the majority in comparison to other religions. Even if they are the majority as they claim, this is still not an evidence that they are upon the truth. Allaah says (which means): {But most of mankind know not}[7:187]. Allaah also says (which means): {And if you obey most of those on the earth, they will mislead you far away from Allaah's path.}[6:116]. In another verse, Allaah further says (which means): {And most of mankind will not believe even if you desire it legally.}[12:103]. Therefore, the majority of people have gone astray from the right path. It is confirmed that the Prophet described his nation as being small number in comparison to non-Muslims. He said: "You, Muslims, in comparison to others, are like a black hair in the skin of a white ox, or a white hair in the skin of a black ox." [Al-Bukhari]  Another narration reads: "You, Muslims, in comparison to disbelief … [that's to say in comparison to the non-Muslim nations]."

Allaah knows best