Take Mr. Biden’s decision to overturn the Mexico City policy and call for repeal of the Hyde Amendment. Those policies long prevented taxpayers from funding—and thus being complicit in—elective abortions, overseas and at home. But protecting pro-life people’s right to dissent isn’t the most important reason to oppose abortion funding. The complicity matters because killing unborn babies is profoundly unjust. Conservatives’ refusal to fund abortions communicates that message, which is why the left has turned against popular consensus to oppose the Hyde Amendment—including Mr. Biden, who caved to pressure after supporting Hyde for 40 years.

Or consider Mr. Biden’s approach to gender ideology: “Transgender equality is the civil rights issue of our time. There is no room for compromise when it comes to basic human rights.” By his own admission, he’s not looking for common ground—as reflected in his day-one executive order directing that all citizens, including children, be recognized by their declared gender in federal law.

Conservatives have no choice but to push back. And here, too, the terrain to defend is not primarily about religious liberty, or religion at all. Children feeling discomfort with their bodies shouldn’t be told the lie that they’re “trapped in the wrong body,” nor should adults pump them full of puberty-blocking drugs and cross-sex hormones. Doing so is unethical medicine, a violation of the bodily integrity of children—whether or not those kids or their parents or doctors are religious.

Likewise for preserving female athletics. Secular and religious high-school girls are equally interested in the fair competition that is undermined when boys who “identify” as girls compete against girls. And it isn’t only churchgoing girls who prefer not to share showers and locker rooms with boys. The main reason for separation is not religious qualms, but biology.

The left would love to frame these issues as if they pitted reason and science against superstition. But on all of these issues social conservatives are on the side of the biological facts. The Democratic Party and the left are the science deniers. To be sure, Christians believe the historic religious teachings on these issues are anything but superstitious: The scientific point of view confirms the biblical teaching that humans are created male and female. It requires no faith to know that a boy who “identifies” as a girl isn’t one and shouldn’t be allowed into private female spaces.

Likewise, although Christians believe all people are made in God’s image, it requires no faith to see that an unborn child is a child. Even atheists post ultrasound images of their kids. When they shoot off pink confetti for the gender reveal, they reveal that they know biological sex isn’t “assigned at birth.” It’s time to make the left follow the science. Conservatives shouldn’t frame these as sectarian religious issues, litigating them purely in terms of religious liberty.

Even when it makes sense to argue these issues as matters of religious liberty, conservatives shouldn’t pretend to be agnostic about the truth of our perspective. We’ll have the best shot at winning fights over abortion restrictions or child sex-change procedures when conservatives are willing to assert that their beliefs are true, not merely protected in law.

Lawyers will have to make specifically legal arguments, rooted in the First Amendment, the Religious Freedom Restoration Act, the separation of powers or the Administrative Procedure Act, none of which turn on the truth of the belief seeking protection. But the rest of us needn’t speak like lawyers. If we fail to fight back in the court of public opinion against the claim that our beliefs are “bigoted,” we will ultimately lose even in courts of law, where the soundness of our beliefs is supposedly irrelevant. If basic truths of human nature are redefined as religious bigotry, they will be excised from society, in court and out.

The reality is that there is a culture war in the U.S., and conservatives aren’t the aggressors. While the moniker “culture warrior” seems to be applied only to those on the right, we aren’t the ones who imposed abortion on demand up to and even during birth, forced Catholic nuns to pay for abortifacients, redefined marriage, harassed evangelical bakers, or declared it “unlawful discrimination” to refuse to put a confused child on puberty-blocking drugs. These salvos came from the left. Conservatives have been playing defense.

But a strategy based only on religious liberty won’t work in the long run. Americans need to figure out how to coexist peacefully on these issues. But the answer isn’t for our side to forfeit the fight about the truth by pleading only to be left alone.