Here, in chronological order, of Pope that were instrumental for the Church:

440-461

St. Leo I the Great

He stared down Attila the Hun in 452 to prevent the sacking of Rome and later persuaded a Vandal king to spare the people. The first to rule that popes are successors to St. Peter with authority over all the faithful, "he established the pope as someone who could intervene in just about any affair," O’Malley says. He gave the "definitive teaching on the divinity and humanity of Christ," McBrien says.

590-604

St. Gregory I the Great

He’s one of only two "outstanding" popes in McBrien’s ratings, for being a "genuinely pastoral pope." He "saved his people from marauding bandits, sold papal property to help feed them … and set a vision of pastoral care for the church," O’Malley says. The first monk to be chosen pope, Gregory dubbed himself "servant of the servants of God." Yet he also helped launch the evangelization of northern Europe that transformed Christianity.

1059-1061

Nicholas II

The French-born Bishop of Florence became the pope who turned cardinals into kingmakers and cracked down on simony — the buying and selling of church offices. Before the Lateran Council he convened, the pope was chosen by clergy, the faithful and sometimes secular authorities, O’Malley says. McBrien calls Nicholas a bad influence for his imperial pretensions.

1073-1085

St. Gregory VII

He makes O’Malley’s list for consolidating the papacy as a centralized monarchy of secular and religious power. In his heyday, Gregory famously brought German King Henry IV to his knees, making him stand in the snow for days begging forgiveness. Eventually, the pope lost control of the city to Norman invaders and was driven from Rome. But in his efforts to combat corruption and secular interference, he transformed the papacy into a legalistic office, an influence "mostly for ill," McBrien says.

1198-1216

Innocent III

He broadened and deepened the lives of the medieval faithful by approving the Franciscan religious order, dedicated to preaching and healing, and the Dominicans, known for their learning, O’Malley says. He was also known for his "power and his pretentiousness," McBrien says. He promoted the disastrous Fourth Crusade and saw himself as "less than God but greater than humans," McBrien says.

1503-1512

Julius II

He was "the antithesis of the Apostle Peter" and McBrien blasts him as a "warrior pope" known for arranging to sell indulgences to pay for building a new St. Peter’s Basilica. (Indulgences are a way forgiven sinners can shorten or escape the punishment of purgatory.) That helped provoke Martin Luther’s Protestant Reformation. Even so, O’Malley cites "his legacy of beauty." Julius persuaded Michelangelo to paint the Sistine Chapel, commissioned paintings from Raphael and assigned architect Bramante to design the new St. Peter’s.

1534-1549

Paul III

He convoked the Council of Trent, which organized the church in response to the Protestant Reformation. He cracked down on clerical abuses and established formal seminaries to train priests, O’Malley says. Paul excommunicated Henry VIII rather than grant him a divorce, thereby isolating England from the Catholic world. He also founded the "Roman Inquisition" to enforce doctrinal purity.

1846-1878

Blessed Pius IX

His was the second-longest pontificate in history, including eight years he refused to leave the Vatican as a political protest to losing the papal state to the new unified Italy. He sparked a spiritual revival in 1854 with the declaration of the immaculate conception — that Jesus’ mother, Mary, was born without sin. And, in 1870, he pushed through the doctrine of papal infallibility in faith and morals, making the papacy the watchdog, teacher and final arbiter of doctrine, O’Malley says. But McBrien also cites him for condemning free expression and approving the secret baptism of a kidnapped Jewish boy.

1878-1903

Leo XIII

Dubbed the "workers’ pope," Leo laid the groundwork for Catholic social thought and the church’s response to modern economics and industrialization, McBrien says. Generations of popes dedicated their writings to Leo on anniversaries of his encyclical on the subject.

1958-1963

Blessed John XXIII

John was "the most beloved, ecumenical and open-hearted pope in history," McBrien says. In the 1960s he called the Second Vatican Council, turning the church to the 20th century with a broad series of reforms such as using the local language for the liturgy instead of Latin and issuing a document on the fundamental rights and dignity of all human beings. His encyclical on peace was addressed to all people of good will — within and beyond the Catholic Church. This pope renewed the faith and reached out to the world, "radically changing relationships with other religions," O’Malley says.