Now just a minute, you cannot have only {{ .Content }} in your index.html template and expect to get a valid HTML page in the browser.
Please have a look at the repository of the Ananke theme that is used in the Quick Start Guide to see how you need to structure your Hugo project templates for valid HTML.
In addition to making sure you have valid HTML, you also need to be careful with spaces or indents in your content. I noticed your text is indented by 8 spaces; Markdown interprets 4-space indent as ‘this is a code block’.
Thanks @alexandros, I had my own theme in another hugo project that I created for learning purpose, markdown was not working and so I created a simple version for troubleshooting, but thanks for raising this up =)