"My prayer is that they would be one, even as we are one." John 17:17
Certainly the Protestant Reformers did not want to start their own denomination. Most- Luther, Bucer, Calvin, wanted reforms for the church.
Most believed the medieval church was corrupt. There was simony (selling of offices- or appointments to religious positions for money), nepotism (giving special appointments to relatives), all kinds of sacerdotalism- so that the priest alone could absolve sins, the priest alone could read scripture (and only in Latin), the priest alone could say how many years someone would spend in purgatory (or getting out of it), and the priest alone ruled the church (no elders, deacons, boards).
In hindsight, we may think such efforts would be futile. Nowadays it seems that the differences (between Roman Catholic and Protestants) are sharp. But before the Council of Trent that condemned the Protestants (yet accepted some of their ideas about stopping corruption), there was a real chance at reconciliation. Many thought that the oneness of the church was important until Trent (1545-63).
The Diet of Speyer 1526 was held by the Holy Roman Emperor as a way to make peace. He was out of favor with the Pope, and the Turks were pressing upon Austria. Charles V wanted to have a peace so both sides of the reform issues could fight the Muslims together. His solution was to suspend the edict of the Diet of Worms (condemning Luther and his followers) and that each prince could declare their area's religious preference. So, Saxony and Hesse could be Lutheran.
So there were several discussions or colloquies to try to bring the Roman Catholics and Protestants together. Hagenau in June/July 1540; Worms November 1540; and Regensburg 4/1541 (and the Regensburg book of Bucer and Gropper). In the end Regensburg came up with a common article of Justification by Faith. The reaction was that there was vast disappointment that common agreement on a union in all things in the church was not reached. The emphasis was not on the one point of agreement. Johannes Gropper (Catholic) and Martin Bucer (Lutheran then Reformed) recognized each other as Christian and had mutual respect for each other. There was another colloquy in Regensburg in 1546 but the catholics rejected their compromise and emphasis of justification through Christ in the sacraments.
There was also the Peace of Augsburg 9/25/1555 between Charles V of the Holy Roman Empire and the Schmalkaldic League (German Protestant princes). This basically allowed rulers (and thus their regions or subjects) to choose between Roman Catholicism or Lutheranism. Calvinism was not allowed in the Holy Roman Empire though there were certainly elements of it- Prussia was primarily Calvinistic. One of the problems with this peace is that many who were not Lutheran or Roman Catholic still found themselves in danger of being charged, tried, and condemned with heresy (Zwinglians, Calvinists, Anabaptists). It was only after the Thirty Years War and the Peace of Westphalia of 1648 that there was religious peace. Religious Freedom and the sovereignty of states (including the Netherlands) was recognized. But with all the fighting came disillusionment with state-run religion that still effects Europe today.
My contention is that this Thirty Years War was caused mostly by those who wanted power and physical resources (land, castles, money). True religion goes against such things. Jesus would rather have the cross than cause a rebellion against Rome or the Bureaucracy of the Jewish rulers. His desire for oneness was not only not heeded, it was totally forsaken and rebelled against. Luther said, "Love God and do what you will." But then he did not show that love toward those who differed with him (nor did the Pope, nor did Zwingli).
Today, the church is again divided. It has become so argumentative that many would rather not be affiliated with any denomination lest they be 1) Seen as contentious; 2) Have someone else encourage them to think differently in the name of unity. We say we value truth over love. But love is the truth. God is love as well as light. They are not incompatible. When we choose only "truth" or "light" we fall into the darkness of disunity and even hatred of those who will one day be with us in heaven- who believe in Jesus Christ as their Lord and Savior. If the church cannot teach us to be united above our differences, then it is no better than the worst political divisiveness.
One of the great things that has happened were the ecumenical councils that began in missionary efforts (Edinburgh 1910). But Vatican 2 (1962-65) began to recognize Protestants, encouraged the liturgy and the reading of the Bible in the vernacular. In 2015 Pope Francis declared division among Christians as "the work of the father of lies" and declared the oneness and brotherhood of those who believe "whether they are Evangelicals, Orthodox, Lutherans, Catholics, of Apostolic." In 2016 Pope Francis celebrated the Reformation and even had a Vatican stamp made with Luther and Melanchthon kneeling before the cross.
The great sin of our day is not what we believe about justification- or celibacy. The great sin of our day is a lack of love and unity.
Prayer: Lord, let me be a peacemaker. I believe what you said, "blessed be the peacemakers, for they will be called the children of God." Help me to sacrifice my pride and my all to be true to you- in truth and love.