the fastest way to earn bitcoin and the safest is simply by buying the bitcoins and the most rewarding which should be considered the riskiest is GAMBLING with your bitcoins 
buying is not earning... But anyway, I really don't like the idea of gambling to make easy money... Sometimes the result is not in our favor, we release money instead of gaining when we are gambling, so I think it's a big "NO" to earning.
It is upto your option of what type of gambling you are betting, if you are taking sports betting , then their are so many matches whose odd are low but confirm winning is their , in that matches if you bet then their is lot of chances that your bankroll on long term will be high, apart from that if your rank is high then you can join signature campaign, so that while posting you will also earn some extra bitcons.