Secularism is a worthy notion, even if it does end up surrounded in slightly odd perceptions. It is quite common to read accounts of secularist atheists which mention the valuable support and accord of secularist theists but, while I have no doubt there exist such, I expect it is a far less common convergence of commitments. This is one of the oddest perceptions: that secularism is somehow a threat to religious belief. Considering that in general secularism is not just about holding religious belief separate to any political power, but also about enshrining freedoms to believe and practice and religion, or no religion, ‘odd’ barely covers it. Why object to anything that protects your own freedom?
As usual, though, that is not what is going on. Evangelism (or the nagging fear that people elsewhere might not believe what you do) has a much higher incidence among theists than atheists. I was once told by a pair of evangelists at my door that they were as happy as any non-evangelist atheist such as myself to live and let live, to let people be. This seriously invited the question of why they were waving their Bronze Age documents in my face. The point here is that there is a tendency among people who believe in absolutist notions (which is inextricably linked with most religion) not just to want their own freedom to carry on such belief, but freedom to diminish and outlaw any dissent. It has been said that wherever religion acquires political power, atrocity, or at least oppression, follows.
Very few, if any, theists would accept this. But thankfully there is a less disputable reason to mock general religious attitudes towards secularism. In most cases wherever secular systems end up holding sway either formally or informally, religious observance tends to decline noticeably. A superficial reading of that sort of statistical conclusion is that secularism is therefore some kind of attack on religious belief, and this is why it must be opposed. But there is a much better interpretation of why this happens. And, possibly, why it does not happen in the few anomalous cases. When religion loses its mandate to inveigle its way into all aspects of life, to indoctrinate at all stages, and alternatives are allowed, that section of the population who observe religion out of indifference, or peer pressure, outright fear, or any other reason aside from actual faith, fall away from it. The anomalies presumably occur where one or more of these other reasons still possesses enough force to coerce the agnostic middle into identifying as a theist.
So secularism is a problem for the theist, not because it undermines any of an individual’s beliefs or (within legality) right to practice. It explicitly protects that. It is a problem because it detracts from religion’s own inherent need to proselytise its own rectitude. Or, rather, its natural desire to impose its own structure on others and not to tolerate difference, dissent or criticism.