An invalid baptism was on indicia of heresy. It was not proof that one was a pagan or a heathen.
Mormonism isn't in a special category. It's clear for Roman Catholics. It is by a valid baptism that a person is initiated into a Christian life. If you have a valid baptism, you're a Christian religion, if you don't, you aren't.
Mormons, Oneness Pentecostals and Christadelphians are the three I can think of off the top of my head that do not have valid baptisms, and therefore are not Christian religions. All because of their rejection of the Triune God, Who is the Christian God.
That isn't to say that people who belong to these religions are not seeking Christ. They do so in a non-Christian framework.
If one wants to re-do the theology, that's perfectly acceptable. That's what religions do. But I think to be intellectually honest about it, one must first make transparent the history of the ideas under consideration. Otherwise, its not theology, it's public relations.
madeleine, please engage directly with the classical Christian terminology of orthodox, schismatic, heretic, infidel, pagan and heathen.
Heresies are addressed as they arise, which is not PR but addressing a heresy. To be a heretic, in Catholicism, you have to first be a Catholic. Martin Luther was a heretic. Thomas S. Monson is not.
Mormonism has never been declared heretical, because first it would have to have been Christian. Strictly speaking, it is not a heretical Christian sect because it isn't Christian and never has been.
Mormonism is not schismatic or infidel. It has a strong pagan influence/leanings and has adopted (or recast) some Christian heresies, creating a new religion. It seems to go purposely out of the way to not adopt any teachings that are apostolic in origin. If the apostolic faith is so rejected at the foundation, it is impossible for the religion to be heretical. If you want to slot it, firmly in one place, I'd say it falls under paganism. The multiple gods doing it in there. Recasting the Persons of the Holy Trinity into paganized gods (and themselves as well), making it more Greek/Roman than Christian. That would make it heathen, being neither Jewish, Christian or Muslim. But that is my opinion, not the Roman Curia.
Ancient heresies, from the Catholic view, were heresies because they deviated from apostolic teaching. Meaning, the group or individuals were at one time *not* heretical. Heresies arise most often by people or groups who focus on one thing, very intensely, removing that *thing* from the context of the whole faith. Creating new explanations, doctrines, etc.
This has never happened with Mormonism. It is not a heresy that arose from the apostolic faith, but is a new religion that arose, using Christian terminology and scripture, but recasting both into a non-Christian context with new meanings.
Catholic theology does not always approach a question by explaining what is, but rather will explain what it is not. What Mormonism is
, is not something I've seen addressed, in an official capacity. What has been addressed is what it is not, the Roman Curia promulgating a decision that Mormon baptisms are not valid...that makes the religion, for Catholics, not a Christian religion.
I haven't seen a St. Irenaeus type "Against Mormonism", though, some of what he wrote in "Against Heresies" can be applied to Mormonism.
"1. Such, then, is their system, which neither the prophets announced, nor the Lord taught, nor the apostles delivered, but of which they boast that beyond all others they have a perfect knowledge. They gather their views from other sources than the Scriptures; and, to use a common proverb, they strive to weave ropes of sand, while they endeavour to adapt with an air of probability to their own peculiar assertions the parables of the Lord, the sayings of the prophets, and the words of the apostles, in order that their scheme may not seem altogether without support. In doing so, however, they disregard the order and the connection of the Scriptures, and so far as in them lies, dismember and destroy the truth. By transferring passages, and dressing them up anew, and making one thing out of another, they succeed in deluding many through their wicked are in adapting the oracles of the Lord to their opinions. Their manner of acting is just as if one, when a beautiful image of a king has been constructed by some skilful artist out of precious jewels, should then take this likeness of the man all to pieces, should rearrange the gems, and so fit them together as to make them into the form of a dog or of a fox, and even that but poorly executed; and should then maintain and declare that this was the beautiful image of the king which the skilful artist constructed, pointing to the jewels which had been admirably fitted together by the first artist to form the image of the king, but have been with bad effect transferred by the latter one to the shape of a dog, and by thus exhibiting the jewels, should deceive the ignorant who had no conception what a king's form was like, and persuade them that that miserable likeness of the fox was, in fact, the beautiful image of the king. In like manner do these persons patch together old wives' fables, and then endeavour, by violently drawing away from their proper connection, words, expressions, and parables whenever found, to adapt the oracles of God to their baseless fictions. We have already stated how far they proceed in this way with respect to the interior of the Pleroma." (Against Heresies (Book I, Chapter 8 "How the Valentinians pervert the Scriptures to support their own pious opinions."))
Hope that helps but isn't too harsh for the LDS ears.