Because many believe that it is the truth and the idea of everlasting happiness brings comfort and the love of God guides them. It is all very pleasing!
Christianity itself has its roots in the Jewish faith. There are no popular religions that are based on Christianity, however there are a lot of forms of Christianity such as Orthodox, Methodist, Baptist, and so on.
Chistianity became popular because Rome adopted it.
Christianity
There wasn't a fall of Christianity. It is still a popular religion.
Christianity is popular because it is The Truth and key to heaven.
America
Christianity
Trends come and go, Christianity is at its best when unpopular.
Judaism started out as more popular, then Christianity was created, and it became more popular.
Christianity became popular after Augustus's rule.
Christianity - and the particular denomination is Catholicism
Muslim faith is popular in the Arabian countries and Arabian influenced countries because it has the same theory as Christianity/Catholicism but has a different ethnicity.