78 years ago, a journalist who studied 500 millionaires isolated a key to success we still preach today

In 1939, journalist Napoleon Hill published his masterpiece, "Think and Grow Rich," which would become one of the bestselling books of all time.

He drew on interviews with - and extensive research on - over 500 incredibly rich and successful men to uncover and share their strategies, which he outlines in 13 principles.

One commonality he found was what we now call "following your passion."

Do what you love, Hill says, or else you will lack the energy to become truly successful.

"No man can succeed in a line of endeavor which he does not like," he writes. "The most essential step in the marketing of personal services is that of selecting an occupation into which you can throw yourself wholeheartedly."

Nearly a century later, and this emphasis on passion and enthusiasm is just as - or even more - relevant.

Richard St. John, who conducted interviews with over 500 highly successful people including Bill Gates and Richard Branson before publishing his book, "The 8 Traits Successful People Have in Common," puts passion at the top of his "8 to be great" list.

If you love what you do, other traits required to be successful will come easier, he argues. If you're passionate, you will push yourself to step outside of your comfort zone, you will want to constantly improve, you will continue to come up with new ideas, and you will be persistent through failure.

"Do what successful people do and follow your heart, not your wallet," he writes. "In the end, your wallet will be happy you did." To know if you've found your passion, ask yourself one simple question, he says: "Would you do it for free?" If the answer is yes, you've probably found it.

St. John is not the only one convinced of this secret to success.

"You've got to find what you love," Steve Jobs said during his 2005 commencement address to the graduates of Stanford University. "The only way to do great work is to love what you do. If you haven't found it yet, keep looking. Don't settle. As with all matters of the heart, you'll know when you find it."

Apple cofounder Steve Wozniak echoed this belief: "If you love what you do and are willing to do what it takes, it's within your reach," the successful entrepreneur said. "And it'll be worth every minute you spend alone at night, thinking and thinking about what it is you want to design or build. It'll be worth it, I promise."

If the circumstances are right for you to quit your job, it can be incredibly rewarding and pay off in the future. For ideas on how best to go about it, take a look at the advice successful people like Richard Branson have for quitting your job.