Why was the council of trent convened?

The council of Trent was the Vatican's response to the mounting following of protestantism (particularly Lutheranism and Calvinism, which were prominent in the mid 1500s when the Council was actually called). Its intent was to reform catholicism in order promote a mass reconversion back to catholicism amongst those who had recently adopted the less conservative ideologies of the Protestant Churches.
Fundamentally, the church had two options which it could conduct to meet the end of reform: incentivize its lost followers to return or punish those who did not align themselves with the teaching of the Church. Eventually, over the course of 15 years of discussion at the Council of Trent, the Church decided to adopt a plan which shared characteristics from both of these approaches.
 To discourage non-catholic beliefs, it created the Inquisition, a group of holy "secret police" who punished non-catholics through bodily torture (this was done in unique ways including crushing the thumbs of and burning non-catholics because the precepts of the catholic church -rather arbitrarily - proclaimed that it was unjust and sinful to intentionally cause someone to bleed, but nothing was stated in the Bible about other forms of bodily harm). The church also created the index of forbidden books, a list of books which were illegal to posses and were ordered to be banned. Most of the books included on the list promoted non-Catholic ideologies (Luther's 95 Theses and Calvin's book on religion were included) however some seemingly innocent books (such as Bocaccio's Decameron) were also included. In this way, the Church established a widespread ideological censorship over much of Europe. The thinking was that if these books were all burned, people would be left unaware of alternatives to Catholicism and no more conversion would occur. In theory, this is logical, however it was nearly impossible to implement such widespread censorship and most books on the list still survived in one form or another. 
Aside from trying to scare Europe into mass reconversion, the Church also changed its principles so as to encourage people to reconvert. This reform included six changes:
1) The Church was the only source of doctrine. Individuals cannot decide upon issues of religious nature for themselves. Seeking to understand spiritual truth individually is forbidden.
2) Church traditions are equally important to scripture (this is an attack at the protestant-specifically calvinist-ideology of sola scriptura).3) Salvation requires a mix of faith and actions (again, an attack at the principle of sola scriptura. This also functions as a means of enticing the poor who want the benefits of church charity).4) The pope is the absolute ruler of the church.5) More importance is placed on the individual sacraments in order to ensure a heightened sense of spirituality of churchgoers and to entice those who subscribe to individualist ideologies. 6) More emphasis is placed on personal mysticism and spirituality, which is aided by educational groups such as the jesuits (aligns with individualist thought).Overall, the Council of Trent was unsuccessful in meeting its goal of reconversion, however it is still epitomizes counterreformation today.

He opposed the ideas of the enlightenment and liberal changes his entire life.