Why is it I get a popup warning saying the page contains secure and nonsecure items?

The reason you get this error is because the secure page you are visiting has hyperlinks and images that are linked from http instead of https. All you need to do is add the https into the links on the page and this will solve the problem.

Change <a href="http://www.domain.com/blog/index.php">Click Me</a>
To <a href="https://www.domain.com/blog/index.php">Click Me</a>
or <a href‌="blog/index.php">Click Me</a>

Change <img src="http://www.domain.com/images/logo.gif">
To <img src="https://www.domain.com/images/logo.gif">
or <img src="images/logo.gif">