They achieved more than they meant, for Newman was propelled by the logic of his arguments into the Catholic Church.I find that interesting. Newman had no intention of becoming Catholic. What he intended to do was to defend orthodox Christianity from liberal protestants. What he found was strong biblical arguments could be countered by other biblical arguments. He didn't feel they were as strong but it was more than that. It was not just that the liberal thinkers had accepted an inferior exegetical position. They were simply in rebellion against God. He could see it but he could not make the case logically.
One common tactic he would use was to show how the liberal ideas he was opposing had consistently been opposed by Christians always and everywhere. But the liberals had a strong reply to that. The reformation. That was a break from what Christians has always taught. If you can break once why can't you do it again? Newman tried to find a principle that distinguished the tradition he accepted from the tradition he rejected. It just wasn't there.
He knew liberalism would lead to atheism. The Anglican faith was being eroded and he could see nothing that was safe from that kind of attack. There is a saying that if you want something then any excuse will do. The sinful human heart would mean even quite weak biblical arguments would be appealing. Conservative protestants could make great arguments but they could not settle any question.
He also came to realize that traditionalism would lead to Catholicism. That is was impossible to embrace the early church and the reformers as well. People argued that modern Catholicism is no better. That no principle can be established that differentiates the changes the church has made from the traditions that need to be kept on a solid foundation.
Much of Newman's contribution came in answering this objection. He pointed out that some changes, which he called developments, were not logically problematic. Like defining the trinity. If you were Arian you would say that the church had fundamentally changed. But really she had just deepened her understanding of what was already part of the deposit of faith.Other changes, like the idea of a merely symbolic presence of Jesus in the Eucharist, they meant that the church has gotten something deeply and dreadfully wrong. That the error was widespread, deeply held, and very serious. Or the new doctrine was wrong. Either way the deposit of faith could not be seen as being preserved uncorrupted.
He argued that Catholicism can reasonable say the changes the church has made were developments and not corruptions. Not that nobody will claim they are corruptions. Just that reason does not prevent us from believing they are developments. He tried to systematize it so it would not be seen as simply wishful thinking. He didn't remove all the subjective elements from the analysis but he was able to show that Catholic doctrinal development is quite remarkable. It shows real signs if supernatural intervention. Not just to have a church survive so long but to be free from the kind of obvious corruptions that one finds in every protestant tradition in almost every century.
Newman was thinking like a Catholic long before he became a Catholic. He believed some traditions are matters of settled doctrine and anyone who rejects them is simply a heretic. A lot of converts find that to be true. As a protestant, I certainly felt that way about Christian sexual morality and about some central doctrines like the bodily resurrection of Jesus. I would never have described them as infallible teachings of the church but that is what I really believed about them. People like me see Newman as a spiritual father.