Meteor Subscribe doesn't update sort order of collection

The publish function determines which records should be synced to the mini-mongo database of any subscribing clients. So sorting the data in the publish function actually has no effect on the client, as the client-side database will likely store them in some other way.

Of course you may want to use sort in a publisher's find in order to limit the number of records to the N most recent - but again this is just a way of deciding which records get synced and not how they are to be stored/used by the client.

Once the records have been synced to the client, it is up to the template code to determine how the results should be displayed. For example:

Template.myTemplate.elements = function() {
  return Posts.find({}, {sort: {createdAt:-1}});
}

Also see the "sorted publish" section of my post on common mistakes.

You didn't posted your template helper code.

When you do return Posts.find() from the helper function, the query should also contain the sort arguments, like as below:

Template.myTemplate.elements = function(){
   Meteor.subscribe('thePosts');
   return Posts.find({},  {sort:{createdAt:-1}, reactive:true});
}