The Council of Trent was called in 1545, long after the Reformation began with Luther’s posting of his 95 Theses in 1517. Some Protestants attended some of the sessions, but by 1545 reconciliation was impossible, polemics were shrill. The Protestant doctrines by then were set in concrete, there were too many intervening events. Luther was nearing the end of his life and would pass away the following year, many princes in Germany were Lutheran, John Calvin had written his Institutes establishing what would become the Presbyterian Church, the Jesuit order had been established to battle for the Pope, and Henry VIII had divorced many wives, beheading some, leading to the establishment of the Anglican Church.
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Medieval popes were reluctant to call church councils, fearful they would infringe on the power of the papal office. Pope Clement delayed endlessly when the Holy Roman German Emperor Charles V pushed for a church council, and his successor, Pope Paul III, cancelled several attempts to call a council for pressing political reasons, including Charles V’s war against the Lutheran Schmalkaldic League of German princes, whom he thought he force to attend if they were defeated in battle. By the time the first session of the Council of Trent was called interest in a church council had waned, consequently only 29 bishops attended initially. Attendance was never overwhelming. The Council of Trent met in three sessions called by three popes from 1545-1563, the last session had fewer than 300 bishops, many from Italy, still a small percentage of the 2,900 European prelates. The rulers of Europe also encouraged or forbade their bishops and abbots to attend, depending on the politics at the time, so the national mix of bishops differed in the three sessions.
The popes determined what would be discussed at Trent, which the attending bishops resented, and though the popes did not attend they insisted on sending frequent directives. One bishop quipped that at previous councils the Holy Spirit inspired from on high, but at Trent the Holy Spirit arrived in the weekly papal mail bag. The pope and bishops were also influenced by the Holy Roman Emperor Charles V as well as the successive kings of France and Spain.
The University system and scholastic and humanistic scholarship flourished in the century preceding Trent, and this directly affected the Council. Although attendance was slim, like Vatican II there were many prominent trained theologians influencing the debates at the Council of Trent. Indeed, at both Trent and Vatican II, the bishops were educated on the doctrinal debates in long educational sessions by these theologians.
Early on the Council of Trent decided that for every matter of doctrine that was promulgated they would afterward decide on an item of ecclesiastical reform. For the most part, each session built on the work of the prior session, and in spite of the fact that two generations of bishops and theologians participated in the three sessions, the result was a coherent statement of doctrines that would provide a good foundation for the Catholic faith and Vatican II.
THE COUNCIL OF TRENT IS CONVENED, DECREES ON SCRIPTURE AND TRADITION
So many years and popes and kings and emperors had passed since reformers first called for a church council to respond to the challenges of the Protestant Reformation that there was very little enthusiasm for a church council, only a handful of bishops attended the opening sessions, mostly Italian, some German, and a few French and Spanish bishops. Although is doubtful that Luther would have been conciliatory had the council been called earlier, he was in poor health by the time Trent was called, and he died shortly after Trent convened. The Catholic Church was now facing the second generation of Protestants, Protestant theology was set in concrete, which meant that a reconciliation with the Protestants would be next to impossible. Furthermore, in England and in Germany, many clergy had married, and many nobles and kings had gained vast wealth through seizing church property, and if they renounced Protestantism they might need to give up this vast wealth, and that was not going to happen. This is a most depressing thought, that perhaps justification of theft on a grand scale helped to solidify Protestant theology.
But many hoped for reconciliation. Pope Paul III instructed that the council the council could determine right doctrine, but the council could not criticize individuals. In O’Malley’s words, “only the doctrinal errors of the Reformers could be condemned, but not the Reformers themselves.” The Council of Trent was not intended to be a polemical council. Indeed, some of the bishops and theologians attending sympathized with the Lutheran positions.
Travel was slow, land travel was by horseback and carriage, unless you wanted to walk, so it was not unusual for delegates to regional councils to arrive over several months, and although attendance was never high in the first session of Trent, more bishops, abbots, royal envoys, and theologians arrived. The theologians who arrived were scholastically top-notch, so much so that the decrees issued by all three sessions of Trent would deeply influence Catholicism up to the present day. Vatican II elaborated on the decrees from Trent, but did not radically alter them.
A small incident in the beginning of the proceedings highlighted that the bishops attending were not sheep. The opening decree read, “This holy council of Tent, lawfully assembled in the Holy Spirit.” A motion was introduced that would amend this to include “representing the universal church,” as was done in the Councils of Constance and Basil. This phrase had not been included because the papal legates and the Pope himself feared a runaway council that would defy the authority of the Pope.
The papal appointed President of the Council Del Monte let it slip that the Pope needed to be consulted on this proposed changed. The bishops were not happy, verbal fireworks erupted. Only a third of the delegates voted in favor of the change, but “the vote was a wake-up call for the legates – the assembly would not meekly follow their lead.”
The Council had to decide on the agenda. Everyone agreed the Council of Trent had to address both questions of doctrine and reform of church practices. What should come first? Did the church abuses cause the problem of the Reformation? Or did bad doctrine cause the abuses? After debate, the council decided “almost unanimously to treat dogma and reform in tandem. Every decree on dogma would be accompanied by a decree on reform.”
The council delegates were pleased, the papal legates were pleased, but Pope Paul III was not. It took a month for the papal legates to convince the Pope in Rome to accept the principal of parallel decrees. This acceptance by the Pope that this would not be a rubber stamp council ensured that Trent would be an effective council.
Another reason Trent would be an effective council is, like Vatican II, it was first an educational council. Before the bishops and theologians could issue effective decrees on doctrine, they had to first educate themselves on the doctrinal issues that lie at the heart of the Reformations. Many theologians were monks, and few bishops and theologians had direct knowledge of the writings of the Reformers.
In matters of doctrine the council first appointed theologians to draft a series of questions regarding the doctrines of the Reformers. Then there would be debate defending and/or opposing these doctrines, debate that often days or weeks, first by the theologians, then by the bishops. Then a draft decree was formulated, which was debated, often redrafted, until a final vote was taken. 
 John O’Malley, Trent, ‘What Happened at the Council” (Cambridge, Massachusetts, The Belknap Press of Harvard University Press, 2013), pp. 1-19, 78-102.