I believe it was Pope Blessed John XXIII.
Catholic AnswerThere have been many, many popes who have reformed the Church to one extent or another. There have been some very famous Popes, known for their reforming work. For instance, Pope Pius X did a great deal of reformation before he died. He lowered the age for First Holy Communion to 7 or 8, he reformed the Breviary, he made updates in the Missal. He fought many of the errors with which we are still plagued today, such as Modernism.Many of the Popes involved in the Council of Trent, nearly five hundred years ago had a profound effect on the church, particularly Pius IV, and Pius V, who implemented the reforms called for.
Pope Blessed John XXIII called the Second Vatican Council together, and Pope Paul VI oversaw it and started its implementation. Popes John Paul the Great, and the current Holy Father, Benedict XVI, who is still implementing the reforms of Vatican Council II are all considered reformers.
Erasmus advocated reforming the catholic church
The Reformation was devoted to reforming the Catholic Church.
The pope in Rome is the leader of the entire Catholic Church.
There have been untold leaders that have reformed the Catholic Church in the past 2,000 years. The very nature of the Church is to be reformed as it is composed entirely of sinners (saving Our Blessed Lord, and His Mother). So to get a specific answer to your question, you are going to have to narrow it down to a time period, and perhaps a country.
Huldrych Zwingli was the leader of the Protestant Reformation in Zurich. He was a Swiss priest who believed in reforming the Catholic Church through his teachings and writings. Zwingli's ideas laid the foundation for the development of Protestantism in Switzerland.
The Catholic Church did not exist in 4 BC.
The Roman Catholic Church
The pope is the leader of the catholic Church on earth.
Pope Francis is the supreme leader of the Catholic Church.
The leader of the Catholic Church is the pope. Today (2011) that is Pope Benedict XVI.
Erasmus advocated reforming the catholic church
The pope in Rome, today that would be Pope Benedict XVI, is the leader of the ENTIRE Catholic Church, even in Russia.