[5] In 1574, he founded a group charged with deepening Christian faith and devotion; this foundation was part of the wider movement of the Counter-Reformation.

He became interested in the reforms instituted by the Council of Trent, and he proposed a new congregation of secular priests to convert sinners and to restore Church discipline.

The pope, encouraged by the cardinal protector Giustiniani issued a papal decree approving the union of the Lucca Fathers with the Piarists of Saint Joseph Calasanz.

While ultimately ineffective, their efforts forced John Leonardi to spend most of the remainder of his life outside Lucca,[5] with special exceptions granted by its government as he was under the protection of the pope.

In 1596 Pope Clement VIII nominated Leonardi as Commissary Apostolic for the reform of the Benedictine monks of the Order of Monte Vergine and in 1601 their Cardinal Protector appointed him to carry out a similar work among the Vallombrosans.

[8] The church of San Giovanni Leonardi in Zone XV Torre Maura in the east of Rome is named in his honour.