It's not the Christian Republic of America, so why is it those who are religious can hold the bodies of secular women and of other religions hostage?

Because this is a very, very religious country, despite all pagan and nihilistic signs. This country self-identifies as roughly 30% evangelical right off the bat. Those are what we Catholics and Protestants call the "serious" religions. Fanatical, perhaps.

30% isn't too too much until you discover it's the single largest identified religious group in the country, followed by the more conservative factions of Christianity. (https://www.pewforum.org/religious-landscape-study/)

They may be in the minority of the US, but their power is strong for two reasons, I believe. One, they have money. Plenty of it. Two, they have passion. Where more relaxed groups often adopt a live-and-let-live approach, the conservatives seek to control every aspect of life, from conception to birth to education, to healthcare to employment and childcare. Even death. So they fight tooth and nail, feeling they're doing the will of God when they're really juts preserving the block the way they like it.

Social justice is about rewarding all those deserving. Conservative power grabs are simply about keeping the fortress walls strong.