This is a good article from Matt at MandM.
[Secularism] is the view that citizens of liberal democracies may justly support the implementation of a law only if they reasonably believe themselves to have a plausible secular justification for that law. Further, they must be willing to appeal to secular justifications alone in political discussion. The upshot of this perspective is that it is perceived to be unjust to support or advocate for laws for theological or religious reasons.
[...]This raises an obvious question, why the asymmetry? On the face of it secularism appears to privilege secular ideologies and doctrines in public debate whilst relegating religious or theological perspectives to the private sphere. What is the basis for this? Two reasons are typically offered and neither is terribly compelling.
The first is that it is dangerous to allow theological or religious concerns into public debate. Defenders of secularism raise the specter of the wars of religion that tore Europe apart during the 17th century or they mention episodes such as the Inquisition and Crusades, which are said to be consequences of allowing religious reasons to influence public and political life. It is argued that the only way to keep social peace and prevent the kind of violence that Europe witnessed is to ensure religious reasons do not influence public life and that all political discussions take place on secular terms.
[...]The fear of religious wars is not the only argument typically offered for the secular public square. The main reason offered for secularism is that religious reasons are not accessible to all people. Auckland Law Professor Paul Rishworth observes, “some have contended that the nature of religious belief is such that, while it may be integral to individual autonomy and development, it has no proper role in public policy debates and that these ought to be conducted exclusively in secular terms that are equally accessible to all.” [Emphasis added]
Something like this is also evident in defences of secularism. Leading secular Philosopher Michael Tooley states, “For it is surely true that it is inappropriate, at least in a pluralistic society, to appeal to specific theological beliefs of a non moral sort… in support of legislation that will be binding upon everyone.”
Ever heard this argument that only secularism is allowed in public? I actually try to respect their standards of evidence, but I draw conclusions that implicate theism. But Matt and Madeleine disagree with me – or at least they say that neither of these two reasons is enough to rule out reasoning based on religious premises. Intriguing, isn’t it?