On Oct. 31, 1517, Martin Luther nailed his 95 Theses, a list of propositions aimed at problems in the Roman Catholic Church, to the door of the university church in Wittenberg, Germany. He wanted to start a theological debate. Instead, he started the Reformation.

As Luther’s teaching reverberated through Europe, the aims of his followers expanded. According to historian Scott Hendrix, Protestants aspired to “re-Christianize” Europe.

Europe was already Christian, but in the Reformers’ judgment, its Christianity was skin-deep. Europe needed to recover the gospel, the “good news” about salvation through Jesus. The church, in other words, had to be overhauled by the gospel.

In many ways, the Reformation succeeded.

Catholic teaching had obscured the biblical message that salvation is God’s gift to sinners, not something earned by a pious, moral life. Protestant preachers proclaimed free grace.

Protestantism has a future only if we Protestants recover the original catholic vision of the Reformers. We need to take up the project of uniting and renewing the whole church.

Catholic clergy had treated the laity as second-class citizens. Luther proclaimed the priesthood of believers. All Christians are holy. All are priests, he taught. As a result, Protestant teachers diligently instructed the laity, translated the Bible into various European languages and spread literacy so everyone could read Scripture. Bible knowledge exploded.

Another problem: Catholics rarely took the Lord’s Supper, also known as Communion or the Eucharist. When they did, they got only bread. To the Reformers, all baptized Christians should receive both bread and wine, God’s gift to the whole church.

The cultural and political effects of the Reformation were huge. Our world was made, for good and ill, by the Reformation.

We don't even know that we live among ruins.

Without Protestant teaching, there’d be no Charles Dickens (“Great Expectations”), John Milton (“Paradise Lost”) or C.S. Lewis (“The Chronicles of Narnia”). The Reformation also contributed to the formation of the nation-state and global politics.

Despite their achievements, the Reformers failed. The gospel took hold in some pockets, but it didn’t reform the whole church or re-Christianize Europe.

The Reformation failed because it fragmented the Western church. Protestants were forced out of the Catholic Church, and soon Protestants began squabbling among themselves.

Before Luther, the Western church wasn’t perfectly calm. But, as Reformation historian Lee Palmer Wandel has shown, the Reformation produced deeper and more lasting divisions.

In 1500, a “Christian” was a baptized Catholic. By 1600, there were competing definitions of “Christian.” Protestants didn’t view Catholics as “Christians,” and vice versa.

By 1600, European Christians didn’t worship together. Catholics didn’t welcome Protestants to the Mass, and Protestants didn’t share the Lord’s Supper with other Protestants.

Divisions split families. Protestants found Catholic weddings repulsive. To Catholics, non-sacramental Protestant marriages were adulterous and Protestant children were – in effect -- bastards.

In some cases, divisions resulted in violence. During the sixteenth century, Christian rulers killed about 5,000 Christians. Survivors fled or were expelled. By 1600, every sort of Christian was persecuted somewhere in Europe.

The Reformers didn’t set out to split the church. The Reformation began as a retrieval, not a rejection, of catholicity and unity. They wanted to reform the whole Western church.

The Reformers denied that the word “catholic,” which means “universal,” was equivalent to “Roman.” The church is catholic because, in all times and places, there is only one church.

Children in Geneva, Switzerland, where Reformation leader John Calvin lived, were taught to believe in one Catholic Church. Calvin’s Catechism said, “As there is but one Head of the faithful so they ought all to be united in one body.”

Catholics charged that Protestants were schismatics, but Calvin turned the accusation around: “Wolves complain against the lambs.” By demanding submission to Rome, Catholics themselves violated catholicity.

For the Reformers, the Catholic Church wasn’t too visible or united. Rome was so swaddled in the trappings of power that the church’s catholic essence, the communion of saints, was barely visible at all.

Why did things go wrong? How did the Catholic Reformation fracture the church?

Martin Bucer, the Reformer of Strasbourg, Germany, said it best: “Both sides have failed. Some of us have overemphasized unimportant points, and others have not adequately reformed obvious abuses.”

A half millennium on, the church remains broken. We don’t kill each other, but we don’t worship together either. There are still competing definitions of “Christian.”

Protestants and Catholics both confess the same core doctrines of the Trinity and the deity of Christ, but at many points our beliefs diverge sharply.

The worst of it is that we’ve become complacent. We act as if a divided church is normal, even healthy. We’re “house blind” and don’t even know that we live among ruins.

Protestantism has a future only if we Protestants recover the original catholic vision of the Reformers. We need to take up the project of uniting and renewing the whole church.

Some Protestants recoil at talk about reuniting Catholics and Protestants in one church (not to mention the Eastern Orthodox Church and others). They see such a project as an abandonment of the Gospel.

And yet it’s exactly the opposite. The imperative to unity is an evangelical imperative, a demand of the Gospel.

Here’s why. The Gospel begins with God’s promise that Abraham’s seed would unite the nations that had been scattered at Babel. All nations would share in God’s blessing (Genesis 12:1-4). Israel’s prophets envisioned a day when nations would stream to Jerusalem to worship the living God. They would learn the Lord’s ways, and turn their firearms into farm tools.

Jesus fulfills the promise to Abraham and the hopes of the prophets. He broke down the dividing wall between Jews and Gentiles. He died and rose again to form a new, united human race.

The Gospel is good news that God has reconciled the sinful human race to himself through Jesus. It’s also the good news that God has reconciled sinful human beings to one another.

The church is called to be God’s new humanity. Drawing from every tribe, tongue and nation, the church is a living sign of the gospel. By definition, a divided church contradicts the gospel. It grieves Jesus, who asked His Father to make us one as the Father is one with the Son (John 17:20-21).

As long as Protestants remain divided from each other, and separated from Catholics, Orthodox and others, our churches fail to experience the fullness of the Gospel.

Yet reunion seems impossible. Will Protestants and Catholics ever agree on the Pope or Mary or the real presence of Christ in the Lord’s Supper or justification by faith?

We can’t reunite the church by ignoring the past or diluting truth. The church will be reunited through a deeper grasp and more faithful practice of the Gospel we preach, truths Christians confess together.

In fact, we can’t reunite the church at all. Only God’s Spirit can overcome the insurmountable obstacles that stand in the way.

The Reformation recovered the Gospel, but undermined that Gospel by the divisions it sowed.

In other words, the Reformation failed. Five hundred years on, the only viable Protestant future – the only evangelical future, the only future faithful to the Reformers’ vision – is a catholic future.