That is because it's the constant teaching of the Ordinary Universal Magisterium that knowledge of Christ and the Holy Trinity are required for supernatural faith and therefore for salvation. So, for 1600 years, not a single Catholic anywhere taught or believed otherwise, and yet a Jesuit comes along in 1600 rejecting this teaching, and suddenly it's open for questioning? So something taught infallibly by the OUM and the unanimous consensus of the Church Fathers suddenly becomes no longer infallibly taught because a handful of innovators began to question it? Nonsense. While this has NOT been explicitly condemned by the Church since that time, it's objectively heretical without a doubt. And the greatest mistake (by omission) ever made in the history of the Church has been the failure to explicitly condemn this error. This omission is what ultimately led to Vatican II. Father Feeney was the only one who saw and predicted where it was going even before Vatican II happened.
If its really the case that every single Catholic taught this for 1600 years, then I'd agree that that conclusion follows. That seems like a much clearer argument than simply citing the dogmatic definitions (for reasons I've pointed out previously.)
Furthermore, I'm not even sure Vatican II demands you to conclude otherwise, even if you hold to it. It certainly allows for it, but its possible to read Vatican II in a way that doesn't come to that conclusion I think.
The only two pieces of data that come to my mind that could be used to argue against you, and I'm by no means an expert and will need to do more research, but is, I believe Justin Martyr suggests the salvation of Socrates in First Apology, and Augustine suggests in Letter 43 that someone who was born a Donatist, and is sincerely seeking the truth, ought not to be regarded as a heretic. Neither point, however, is a perfect refutation of you, because the first one deals with the Old Covenant situation. The second one deals with a Donatist, who believes in the Holy Trinity, however I don't see why he wouldn't be in a comparable position to an EO or a Protestant (and if I understand correctly, Feeneyites would ALSO say its heretical to say that anyone who identifies as Protestant or Eastern Orthodox could be saved.)