Q: Is Erasing all Content and settings and setting the iPhone as a new one without logging onto iCloud enough before selling it? Will the data be deleted permanently?

I Have recently sold an iPhone, but have not reviewed how to "properly erase the content." Basically all I did was turned find my iPhone off, erased all content and settings. Set up the phone as a new without logging on to iCloud or anything. Erased mall content and settings once again before handing over the phone to its new owner.

The question here is, is the process above enough to get rid of the data permanently, or will the new owner be able to easily retrieve or restore my data easily using "professional programs" as stated on many websites and discussions on the Internet.