I think all religious people should strongly support a secular state. They should strongly support the separation of church and state. I am religious myself and want to point out that such a separation does not equal an irreligious country or whatever believers might be afraid of (and I'm not afraid of being religious in a mostly irreligious country like my own, just speculating).

Separation of church and state means that the individual religions lose their power to conduct political warfare against one another in an attempt to grab national power. These kinds of conflicts should be desired by no one as they are among the potentially most inflammatory and take forever to end while the population suffers. And religion should not be about inflicting suffering on people.

Separation of church and state also consolidates democracy by leveling the playing field, making all actors have to play by the same rules (rational argumentation, appealing to the population by means not rooted within a specialized theological framework), which, if it were a product on a market, would actually be a benefit because the people/consumers could more clearly see and compare the strengths and weaknesses of a proposal. It's similar to the strength of us all speaking English most of the time here on /r/europe and not just expecting everyone else to understand whatever we say in one of the gazillion languages of Europe we might know.

These are just two things, but I want to end by saying that there are also many more reasons that religious people should be strong supporters of the separation of church and state. I know that Europe was heavily marked by the political dominance of religion in the last 2,000 years (and more) and that the connection of religion and state has been the status quo all over the world of millennia, but this does not mean that it is right and it does not mean that religious people should feel compelled to create an organized grab for political power based solely on their theological ideas and practices.

The pluralistic society that works as a marketplace of ideas without giving special preference to one notion above the other is the most free society, the one that grants you the right to be, think, feel, and act as you want (within reason) while ensuring that neither you or the other guy get special dibs to power because of your beliefs—a mechanism that can foster cooperation and hinder persecution.