Germany isn’t even secular as such, there’s a gazillion state churches and world view organisation, organised under public law and having privileges such as sitting on the public TV councils, and even writing their own employment laws. You do have to be compatible with humanism, though, and not in opposition to the free and democratic basic order.

From the reformation to the age of the enlightenment there were first wars, then people could be cast out of a lord’s territory if they were of the “wrong” creed – which was a huge win in terms of religious freedom, before that they often had to face some sort of inquisition.

Catholic areas were of course catholic, in Protestant areas multiple new creeds developed, some accepted by the state, some not so much. Actual religious freedom was introduced 1848, simultaneously the authority to marry was taken away from the churches and put into state hands. Same thing with schools, though confessions still can (and do) have private schools, but it’s all under state oversight.

That whole approach then got firmed up a bit in the Weimar constitution, put into its current organisational form, then the Nazis happened, and then it got firmed up even more in the sense that the state now is now not neutral but actively humanist. (Even if it’s often outsourced to specifically the EKD as they are very good at not arguing from theological principles but speak plain ethics. In practice no law concerning say stem cell research passes without their ok as their reasoning always demands respect) And this humanist orientation of the state also leads to decisions that I think look rather strange from an outside POV, such as at-will abortions not being legal, but decriminalised. The constitutional court really was shouting “you can’t just willy-nilly declare a developing human to not be human” from the rooftops, reminding politicians of the state’s duty to protect life, while also saying “you don’t have to implement that protection with criminal punishment that’d probably be counter-productive anyway, use social and welfare means”.