insert message in every post

Yes. There is a signature plugin that will add a “signature” or in your case a message, at the end of each post (old and new)…here’s the link https://www.dagondesign.com/articles/add-signature-plugin-for-wordpress/

Hmm – that looks cool, thanks. But I think I’d rather have the message at the *top* of every post. Most of my posts are split, and I have a feeling a lot of readers won’t see the message.

Can’t hurt to try, though! ??

As far as I can see, I think you just enter the code wherever you want it to appear – so yours would probably be at the top of the post ??