I don't think there's sufficient evidence to say Roncalli was an antipope. 

Or, alternatively, do some of you reject Father Cekada's position that they were manifest heretics prior to their election?

Another way to word it, is "When was the heresy manifested publicly and pertinaciously?" 

Right.  If, as per Father Cekada, Montini was never a legitimate Pope to begin with, then what evidence to do we have of manifest heresy before he was elected?  Or, if some disagree with Father Cekada, please explain why.

So, as Father Cekada has said, most SVs have abandoned the position that Roncalli, Montini, et al. had "fallen" from the papacy, but instead believe that they were never popes to begin with.

What is the evidence that Roncalli and Montini were manifest heretics before their respective "elections"?

Roncalli was a Cardinal in good standing and Montini an Archbishop in good standing under Pius XII.  Sure, Roncalli was "suspect" of Modernism, but that suspicion was not enough to prevent him from being made a Cardinal by Pius XII.

Just curious.  What is the importance or motivation for trying to figure this out?

I am unsure as to whether or not Roncalli defected from the Faith before or during his pontificate. Any manifest and pertinacious heresy is very hard to prove for him.

In regards to Montini, I tend to believe that he defected from the Faith during his pontificate. When? There are several possible times with 1965, 68, or 69 being key possible years for manifest heresy.

Luciani is also hard for manifest heresy.

Wojtyła, Ratzinger, and Bergoglio are much easier for manifest heresy both prior and during their "pontificates".

Thanks.  Do you know why Father Cekada says most SVs (I know you're a sedeprivationist) don't hold that they fell from the papacy, but were never popes to begin with?

What is the evidence that Roncalli and Montini were manifest heretics before their respective "elections"?

Roncalli was a Cardinal in good standing and Montini an Archbishop in good standing under Pius XII.  Sure, Roncalli was "suspect" of Modernism, but that suspicion was not enough to prevent him from being made a Cardinal by Pius XII.

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This is an excellent question. I didn't have a good answer to this until recently, when I read a fascinating book called "The Counterlife of a Pope" by a man who was one of the Noble Guard under Roncalli.

It is basically a biography of Roncalli describing his open communism from basically his childhood, and his open support of communism. Apparently all of this was public knowledge, and only has been lost due to poor recollection and the fact that nobody has bothered to look into Roncalli's past.

In the early 50s, the Holy Office issued a decree stating that acceptance of communism was a form of apostasy. So, that means Roncalli was a public apostate for nearly a decade before his "election" in 1958.

Just to take one example out of countless in the book, when Roncalli was bishop of Paris (I forget what exactly his position was; I don't think he was the ordinary; I think he was the papal nuntio or something), the head of the Knights of Malta was caught being a member of Fɾҽҽmαsσɳɾყ and fired. Investigating his background, it was seen that Roncalli had been one of the people who had recommended him for this post. Pius XII sent a canon of St. Mary Major to investigate Roncalli and figure out why he had recommended a freemason to this important post. Roncalli refused to speak with this messenger of the pope, but instead had him speak to his secretary. The canon went in to the secretary's office, and found him wearing a clerical suit (in a chancery office in an openly Catholic country!), who began to rant and rave about how awesome Fɾҽҽmαsσɳɾყ was, and how it was the best thing that had ever happened to the human race. The canon listened with his jaw on the floor, simply aghast. When he could finally speak, he managed to stammer, "But isn't membership in Fɾҽҽmαsσɳɾყ forbidden by canon law?" Roncalli's secretary said, "Oh, don't worry about that, we're going to get that changed very soon! How can the Church not support something as wonderful as Fɾҽҽmαsσɳɾყ?"

Now, this is only a private conversation, but the book details the constant, ongoing, public support of Roncalli of freemasons and communists everywhere he went, for his entire life. For example, when he was a young priest in Castellamare(?) he was seen in the streets bringing food and supplies to communists who were fighting with the police in the streets in communist ιnѕυrrєcтισns. When he was bishop, he used the chancery office's printing press to print communist pamphlets, at Church expense.

There is an amusing anecdote by the author himself, that one day he escorted a cardinal to the office of Roncalli and waited outside, as was his duty. When the cardinal came out, he said to the author, "The pope would like to speak with you." The author went in and Roncalli said to him basically, "You are a good young man, and a good journalist, but you have to get with the times. The times are changing, and they are going to be much better. It is good to be a journalist. I read lots of newspapers ..." He pointed to a stack of newspapers on his desk, of which only the top newspaper was visible, and the author saw that was the official newspaper of the Italian communist party, and obviously he couldn't see anything below that, but probably they were communist all the way down.

The author records countless public events in which Roncalli gave public support to communists and freemasons. This really made me see that he was a public apostate, in terms of the Holy Office pronouncement, for his entire life before his election, as well as afterwards until his death. It was really an eye-opening experience, and made me understand what happened at Vatican 2 much better than I did before.

I strongly recommend the book, which is available for free on the internet.

Thanks.  Do you know why Father Cekada says most SVs (I know you're a sedeprivationist) don't hold that they fell from the papacy, but were never popes to begin with?

I do not know this, but would genuinely desire to learn.