If the Council of Trent didn’t “substantively change” theology or practice then they wouldn’t have made decrees. They could have just skipped the council altogether and announced solemnly “carry on!”
In my experience, Protestants rarely discuss Catholic practices, any more than Catholics discuss Protestants and Protestant practices. By necessity it comes up, but it’s hardly a focus in either church. Protestants worship Christ — that is the “justification of their existence” as you so crudely and evocatively put it, where you view the Protestant as justified by attacking your church. Protestants use the term “justification” in a different sense.
What worries me in the Catholic church is a reactionary stance to Protestant Christianity and the drift to stranger and stranger assertions in the face of it. It’s a sort of grand mistake, where the Catholic church has painted itself into a corner. Suddenly, after 1800 years the world was assured that there is a Christian leader, the Pope, who is infallible. If mankind had known that in the 1800 years before, so many heresies and misunderstandings could have been avoided, and so many changes avoided. My ancestors would have really trembled when they took a Pope hostage. What if he solemnly declared them goats or marmots? And oh, when the same family had a Pope from itself, when they won their power struggle, after alliances, kidnappings, wars, bribes, and the like, wouldn’t they have wondered at their own magnificent, sudden, and assured infallibility?
Lord Acton, a Catholic, responded to this assertion of Papal infallibility by saying that absolute power corrupts absolutely. He was right. Your denomination is wrong. The Pope is a man. He is only infallible insofar as God makes him so, and even Christ himself refused to demand action on the part of the Father, saying that you do not put the Lord your God to the test. To me it seems that the Catholic church chose to step off the cliff instead of heeding the example of Christ; I hope it was an arrogant and trusting act and not a cynical or calculating one.
On a personal level, I am a Protestant. I believe it is a man’s relationship with Christ that saves him not what church or demonination he is a part of.
That is the kind of silly oversimplification that is born of ignorance. You must think I know nothing of Trent. Well, you are wrong. As I said, Trent put limits on what a Catholic can hold. So while transubstantiation was the dominant theology of the Church before Luther, it became the only acceptible formulation of Real Presence at Trent. Before Trent there were some 30 forms of the Mass. Trent confined the use to just one rite with some minor variations. There were no standards for ordination before Trent, afterward there were seminaries with universal standards. And on and on. And, yes, Trent condemned Protestant innovations such as sola scriptura and sola fide, both of which has never existed before 1519. So yes, it is necessary for the church to condemn heresies from time to time and that was what was new in Trent.
It still stands that the Catholic faith was not substantively changed at Trent or by Protestantism.