Original sentence: It is celebrated mainly by Christians to mark the birth of Jesus, whose actual date of birth is unknown, but it is also a secular holiday in many countries with small Christian populations, as well as by non-Christian populations in predominantly Christian countries
Edited sentence: Though originally a Christian festival celebrating the birth of Jesus, whose actual date of birth is unknown, it is also celebrated as a secular holiday in many countries with small Christian populations, as well as by non-Christian populations in predominantly Christian countries