According to PCA pastor Wes White, the doctrine of baptismal regeneration is “impossible in the Reformed system.” By noting this, he intends to show that we should reject the doctrine of baptismal regeneration. But if the evidence for the truth of the doctrine of baptismal regeneration is stronger than the evidence for the truth of the “Reformed system,” then the incompatibility of the doctrine of baptismal regeneration and the Reformed system serves as evidence against the Reformed system.Dr Cross does a good job of finding quotes from many early church fathers showing they did believe the doctrine of baptismal regeneration. There is some work to do here because protestants have an amazing capacity to avoid some of the most obvious truths about the early church fathers.
But Newman goes further than this. He finds that showing protestant Christianity in any of it's forms is different from historical Christianity is the easy part. Protestants often fail to contemplate the significance of that. Just like they fail to ask how they can know they have the right flavor of "biblical" Christianity. It is just as easy not to ask if Christianity was wrong once why can't it be wrong now? But to the extent they do think about these questions the answer is mostly a response of, "Sure there are problems but I believe in Jesus and this is the best I can do as far as doctrine goes."
Newman tackles that question. Is there really no Christian faith that can say it has never been corrupted? If that were true it would be a pretty good reason to doubt the divinity of Jesus. Why did He allow His gospel to become such a mess? If the gospel really is the power of God for our salvation then would God not want to preserve it?
Now, in the simplest form, the question of has the Catholic faith or the Orthodox faith changed since the time of the early church has to be answered with a Yes. Certainly the council of Nicaea changed the faith. It clarified the doctrine of the trinity. But clarifying is not a problem. Even teaching a deep and fuller version of the same doctrine is not a problem. We get a problem only when it becomes impossible to hold that the previous teaching was right. Paul never corrupted the teaching of Jesus because it is possible to hold they were both teaching truth. Nicaea never corrupted the teaching of the New Testament for the same reason.
Newman called these changes that don't corrupt the faith "developments". So the question Newman asked could be phrased as:
Can any Christian faith be understood as an unbroken series of developments from the time of Christ to the present?Now Newman knew there would be some subjective judgements involved in answering this question. But he wanted to keep that to a minimum. So he introduced a series of criteria or notes to more objectively determine what is a corruption and what is a development. Things get quite complicated from there. But if you understand the difference between a corruption and a development you can remove a lot of the complexity.
So the attack on protestantism is not enough. Protestants are used to having their beliefs attacked. What is needed is to show the Catholic faith passes the test that you would expect the true faith to pass. That is the test of non-contradiction.