In another thread we are asked if the church should have any role in governing the state. I said no. That is an opinion. Is it not valid because I have not explained why?

Context is everything. Because I know you well enough online and agree with most of what you say, I would trust that as your honest opinion. I happen to agree with your opinion and if we were in face-to-face conversation about it we would be expanding on our common position. Your curt "no" here could kick off further debate and someone could ask you why you said no. If you said "oh, dunno really, I just said it". (which you won't!), you'd be taken for a buffoon. If you said cautiously, "I'd rather not go into it with you because you're Keith and you may be setting me a booby trap" that would be understandable. As long as you're prepared to support your position if asked to, your opinion as an honest broker is perfectly valid. In my opinion. ;-)