Through the influence of the Colonnas, Giulio was admitted at the age of seven to the Jesuit College in Rome, the most respected school in the city. Though he declined to join their order, he excelled in his studies. In 1618, at the age of sixteen, he gave a public lecture on theories explaining Halley's comet, which appeared in that year. He also excelled in theatrics; he was chosen to play the part of the newly sainted Ignatius of Loyola in a religious pageant. He also acquired the habit of gambling at cards, and frequently was in debt. A particular favourite game of his was Hoc, which is also called Hoc Mazarin.