Final answer:
Pope Julius II financed the construction of the new St. Peter's Basilica primarily through the sale of indulgences, which were promised to reduce punishment for sins. This practice brought in substantial revenue, especially during Jubilee years when pilgrims visited Rome in great numbers.
Step-by-step explanation:
The primary method used by Pope Julius II to finance the construction of the new St. Peter's Basilica was through the sale of indulgences. Indulgences were a form of pardon that the Catholic Church sold, which supposedly reduced the punishment for sins. The money collected from these indulgences was a significant source of funding for the massive architectural and artistic endeavors that characterized the renovation and building of the basilica. This practice later became a point of contention, famously criticized by Martin Luther and leading to the Protestant Reformation.
Pope Julius II's ambition to enhance the papacy and his own reputation, alongside the need to repair and renew the dilapidated Old St Peter's Basilica, necessitated a considerable amount of funds. Indulgences, especially during the Jubilee years, took advantage of the large number of pilgrims visiting Rome, seeking the remission of their sins. The funds generated from these sales were directed towards transforming St. Peter's into a grand architectural symbol of the Church's power and prominence in Christendom.