Start MongoDB With Authentication Enabled
Regarding the issue described in above mentioned post I suggest you to check  authentication options into mongodb configuration file.

# Turn on/off security.  Off is currently the default
#noauth = true
auth = true


And after it initialize mongod service using auth mode.

sudo mongod --auth --dbpath /var/lib/mongodb


Auth configuration option should be set to true and accordingly mongo service will start in authentication mode.