Which is to say, most of the time, that it's religious belief defining what's a human being and what isn't
the main challenge is that we don't have anything else, so a lot of people default to this. you could use viability of the fetus, or something else completely arbitrary in the place of religion, but at the end of the day you're still deciding when legal protection of fetus has a line drawn to have abortion law at all. i don't see many non-arbitrary options for drawing that line.
though as you point out, it seems nearly everyone draws it before/up to 25 weeks, after which people generally are against it/don't do it without some unusual complication or circumstance. if that's the observed behavior pattern for vast majority of voters, that's where it would make sense to draw the line for now.
So unless someone can clear it up for me, a great deal of opposition to Roe v. Wade seems to me to be founded on an opposition to freedom of religion
that's probably most of the opposition to it, and it's wrong. the real problem with roe is the implication of judicial control over this stuff + bad reasoning inconsistent with the case. that's why this now goes to state legislatures, though i'm disappointed in some states, to put it mildly. but when legislatures don't represent what their states want, they should be replaced.
and on an opposition to the separation of church and state
i don't think a coherent religious position could justify abortion bans by state. especially not if they are arguing "separation of church and state". the state isn't forcing abortions (at least not us, china has actually done that). it's the religious position trying to enforce its desire/control other people in this case, using the state as a lever for that control. that is exactly the opposite of "separating" church and state. i have no more patience for classic religious institutions trying to control people with bad government policy than i have for the church of woke.
Am I missing something? Once again, I'm reminded of Margaret Atwood's dystopian vision of America, not merely as a totalitarian state akin to Stalinist
the correct reason to reverse roe and put this in hands of state legislatures is consistency with constitution/design of government/separation of powers. to illustrate why this is important, consider the reverse of the scenario we have now: very religious republican (or otherwise) presidential office + sympathetic courts.
using the same process that led to roe, there's nothing stopping such an arrangement from issuing a *federal* abortion ban and then having courts uphold it, like a reverse roe. yes, this violates constitution and pushes power to judicial/executive that is defined as the purview of states. yes, this moves policy decisions further away from the voters' control. and yes, this crushes voter option to as-easily relocate to governance with which they agree. but roe's ruling did that too, and worse the logic that led to it opened the door for abusive outcomes like this (or similar scope for other issues).
roe was bad case history. not because of religious mumbo jumbo or whatever, but because the courts of the time punted on the question of personhood despite that every abortion law *must* define it, because they weirdly affixed "privacy" as reasoning which should be irrelevant to the important legal question, and because they ignored how the country's design delegates powers to various branches/levels of government. it's bad process. even if you like the outcome of that bad process in one case, it leads to major issues over time.
if you don't want a bible thumper rolling into office in one of the next elections and implementing an executive order banning abortions that a friendly/packed court will uphold as "constitutional", you probably shouldn't want a nonsense ruling wrt privacy that ignored the fundamental question of the case + constitution to be considered good law either.