From his childhood he was noted for being modest as well as for his self-denial and his great diligence in his studies. He was obedient to his parents though when he perceived he was disobedient he begged their pardon and would either kiss their hand or throw himself at their feet. He often visited the church of Saint Francis Xavier as a child. In 1716 he desired to become a Jesuit but his father objected due to his age and directed him to learn law instead; the beatification of John Francis Regis was also an influence in his decision. He studied jurisprudence and earned his doctorate in civil and canon law in 1722. Sarnelli became quite successful and was enrolled in the Congregation of the Knights of the Legal and Medical Professions directed by the Pious Workers of Saint Nicholas of Toledo. One of the rules of this association was visiting the sick in the Hospital of the Incurables. It was while tending to the ill in the hospital that his call to become a priest developed to the point he could not ignore it.