Many religious people evaluate public policy on the merits and reach the same public policy conclusions as non-religious people. We non-religious folk are thankful that so many religious people have a secularist orientation. We lobby together for our shared public policy goals. So why would anyone publicly argue against religious belief? Isn't religious belief the wrong target?
The ongoing Republican Party effort to defund Planned Parenthood illustrates why publicly arguing against religion is what we should be doing. Almost 100% of the slander campaign against Planned Parenthood originates with religious institutions. Some of the people who actively join this campaign against Planned Parenthood are not themselves religious. These people are often career Republican party loyalists who probably could not care less if Planned Parenthood was funded or not and will advocate for whatever is popular among rank and file Republicans. If we experience another government shutdown over defunding of Planned Parenthood, as Republicans are currently threatening, then everyone who voted for the majority Republican Congress will share responsibility for the negative consequences.
As for the religious institutions and believers who have a secularist outlook, they have a secularist outlook despite their religious belief, not because of it. They may self-claim that their secularist outlook is rooted in their religious belief, but then they are mistaken. If they abandoned their religious belief they would still have a secularist outlook. To the extent abandoning religion has a political impact, the impact will be to move public opinion further towards secularism.
However, it is not any political calculus that justifies criticism of religious belief. Religious belief and the campaign to defund Planned Parenthood both merit criticism because they are both built on false assertions and false premises. To criticize religious belief only when it is anti-secularist is to politicize criticism of religious belief. It is wrong to politicize everything this way. There are such things as true and false, better and worse, good and bad. Politics is important, but merit is even more important. After all, good politics is built on merit. We should not limit ourselves to one, we can and should argue both ways.