Answer

My impression is that the Sages assumed that every person is a believer, and heresy is a surrender to the evil inclination. Some people think this even today, although in my opinion that is not the case in most instances nowadays.
As for someone who says there is no resurrection of the dead, that does not necessarily teach us that this is something voluntary. What it says is that someone who says this is outside the bounds of traditional faith, whether this is by his own will or not. Rabbi Chaim of Brisk said that an apikoros under compulsion is still an apikoros. Still, it is true that it is hard to assume that a person who is compelled to think something would lose his entire future (the World to Come — “whoever says there is no resurrection of the dead has no share in the World to Come”) because of that.
I will just note that, as I understand it, with someone who says there is no resurrection of the dead, the problem is not necessarily the content that he does not accept, but the fact that he does not accept the tradition that teaches that there will be resurrection of the dead (because it is not written explicitly in the Torah, but emerges from unusual interpretive readings, such as: “Behold, you are about to lie with your fathers, and this people will rise up and go astray…” ). The importance is not belief in resurrection of the dead as such, but the fact that one accepts the world of rabbinic interpretation in general.
In any case, the Radbaz writes in a responsum that compulsion in matters of belief is compulsion in every respect, and therefore, in my assessment, someone who is truly compelled will not be punished.
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Questioner:
Regarding beliefs in the words of the Sages, they were speaking specifically about believing that resurrection of the dead has its source in the Torah. It does not seem that the goal is to believe in the authority of the Sages. And the problem is twofold: the Sages’ interpretive readings on this subject, which try to show that belief in resurrection of the dead has its source in the Torah, are not convincing; and that makes their demand to believe it even less understandable.
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Rabbi:
I did not understand the question. After all, that is exactly what I wrote: that the Sages saw the severity in someone who does not believe in resurrection of the dead as lying in his rejection of belief in rabbinic interpretation in general. To believe in interpretive readings means to believe that the interpretation reveals what is in the Torah, that is, that it is true. This is precisely belief in the authority of the Sages. The demand to accept their authority is stated precisely where the interpretation is perceived as dubious. In a well-grounded interpretation, that trust is needed less.
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Rafi:
The very fact that it is agreed that a child captured among gentiles is not included among those who lose their share apparently proves that compulsion in matters of belief is considered compulsion, and if so, why should one who was compelled in his beliefs be excluded from this, so it apparently follows as the Rabbi said