Jupiter-puer was, according to Alexander Hislop

Alexander Hislop

Alexander Hislop was a Free Church of Scotland minister infamous for his outspoken criticisms of the Roman Catholic Church. He was the son of Stephen Hislop , a mason by occupation and an elder of the Relief Church...

's book The Two Babylons

The Two Babylons

The Two Babylons is an anti-Catholic religious pamphlet produced initially by the Scottish theologian and Presbyterian Alexander Hislop in 1853. It was later expanded in 1858 and finally published as a book in 1919...

, a boy-saviour deity

Deity

A deity is a recognized preternatural or supernatural immortal being, who may be thought of as holy, divine, or sacred, held in high regard, and respected by believers....

 worshipped in ancient Rome

Ancient Rome

Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

. Hislop writes that he was depicted as sitting on his mother's lap, supposedly inspiring the image of the Virgin Mary and her son, Jesus

Jesus

Jesus of Nazareth , commonly referred to as Jesus Christ or simply as Jesus or Christ, is the central figure of Christianity...

, in Roman Catholicism.

Apart from Hislop's writings, no sources confirm the existence of the cult of "Jupiter-puer".