Christian fundamentalists plot against the US Constitution

[TheRocketSurgeon]

This is why to fear them:

Your response creates an interesting set of conundrums for atheists.

Firstly, your view that "If Christianity is true, then it still should not influence culture, as this is a secular nation." This betrays a fundamental misunderstanding of what secularism is, and its status in the light of the truth of Christianity. Secularism does not entail that religion should have no influence on culture. Rather, strictly speaking, secularism entails that religion should have no influence on the state. This means governance should not be influenced by religion. It says nothing of culture in general, and secularism of the state is perfectly compatible with a highly religious, and highly-religiously-expressive citizenry. So you're operating under an idiosyncratic definition of secularism.

Second, if Christianity is true, then it must be true that there is an afterlife, so to speak. And one's well-being in the afterlife and relationship with God is of utmost importance. Someone who concedes that Christianity is true (even for the sake of argument), will have to concede (once again, for the sake of argument) that the latter two theological notions follow. In fact, one might make the claim that it is an ethical obligation to care for one's afterlife and relationship with God. Thus for an atheist to hold that Christianity is true AND Christianity ought not to have any influence on society is unethical and irresponsible towards one's citizenry, to the same extent that preventing people from access to healthcare and freedom, and the pursuit of happiness would be unethical and irresponsible in our current system.

So there are two big problems for your view: Secularism doesn't entail what you believe it does, and you are endorsing an unethical and irresponsible position that harms the citizenry. What can you do to resolve these two problems? Here are what seems to be your two options:

1) You either affirm the view that religion should have no influence on culture (perhaps you can call this view hypersecularism, as it is much stronger than conventional secularism), or you can reject the view that religion should have no influence on culture, and endorse ordinary secularism.

2) You have to either reject ethics (in which case you are permitted to harm the citizenry by constraining Christian cultural influence even if it is true), or you can affirm ethics and allow a cultural place for Christianity, given its truth.

There are more questions to raise, obviously. One concerns the worth of secularism in a world where Christianity is true. Another is the question of the reason and evidence motivating secularism in the real world. Yet another is the question of what normative standards are being assumed without argument when one says Christianity ought not x, culture and society ought to y, etc. 

I replaced "Christianity" with "Islam" and "Christian" with "Muslim" in my reply to him, to show him why that statement is Not Okay™.