“As we approach the year A.D. 508, we behold a mighty crisis ripening between Catholicism and the pagan influences still existing in the empire. ... The conversion of Clovis is said to have been the occasion of bestowing upon the French monarch the titles of ‘Most Christian Majesty’ and ‘Eldest Son of the Church.’” Daniel and the Revelation, 1944 edition, page 271.

“This title… owed its origins to the long, and distinctive, relationship between the Catholic Church and the Franks. France was the first modern state recognized by the Church, and was known as the ‘Eldest Daughter of the Church;’ Clovis, the King of the Franks, had been recognized by the papacy as a protector of Rome’s interests.” Wikipedia article “Style of the French Sovereign”