`FeatureContext` context class not found and can not be used.

I'm new with symfony2 and i wnat to test my app using behat but when I followed the official install in this link http://ift.tt/1Ie7c26 i got this error : [Behat\Behat\Context\Exception\ContextNotFoundException]
FeatureContext context class not found and can not be used.
it was just after the first line and when i ran the bin/behat command.