A distinctive doctrine? I suppose the doctrine being "There is no knowledge about God". It kind of goes together with lacking a belief about God.its got ism at the end of athe for a reason. it's a part of something.
Secularism has nothing to do with lacking a belief in God. That's just separation of Church and State. You don't want a government telling you what you have to believe. You don't want the government telling you that you can't believe in God any more than I'd want the government telling me I had to believe in a particular God. So secularism is a good thing for both of us,