Early on, Nike's billionaire co-founder Phil Knight had people telling him he wouldn't make it.
Yet, despite the haters, he still steered Nike to become the world's No. 1 athletic-shoe company, with a brand so powerful it can be recognized by a checkmark.
Knight recently chronicled his journey of turning Nike into a $91 billion company in his memoir, "Shoe Dog." In an interview with CNBC's Jim Cramer on Thursday, Knight said that many doubted his ability to succeed.
"I've had a lot of people tell me, 'We ... think he's not going to make it,'" Knight said.
He always knew he could fail, but his optimistic outlook helped him persevere, he said. Building Nike was the most fun he ever had. His team had a cause, and they believed they would succeed.
Any adversity that was thrown at them, they knew they could overcome.
"We knew we could fail, we just didn't think we would," Knight said. "We loved doing what we were doing, and we loved each other."
In 1963, Knight co-founded Nike under the name Blue Ribbon Sports with a $500 investment from his former track coach, Bill Bowerman. He made his first sales by going to track meets and selling the shoes from the trunk of his Plymouth Valiant. As of Aug. 4, Forbes estimated his net worth at $23.9 billion.
Starting a business today is a lot different than it was in the 1960s, Knight said. When he started, he told his 20-year-old sister to go to city hall and get a license. There were no accountants and lawyers involved.
In some ways, though, Knight thinks starting a business today can be easier. He said his biggest struggles growing the Nike brand were not having enough money, since venture capital firms didn't provide funding then like they do today.
However, he worries that people's interest in pursuing entrepreneurship has waned.
"There is a sort of a downtrend, I think, in entrepreneurial spirit," Knight said. "But everything has a cycle. As they tell you in statistics class, the sex life of a flea has a cycle. So I'm optimistic that the cycle will reverse itself here in the future."
Knight was inspired by the entrepreneurial spirit and determination of Olympic athlete Steve Prefontaine, who also trained with Bowerman and became one of America's best-known track and field competitors in the '70s. Knight said he expects the Prefontaine spirit to resonate in this year's Olympic Games.
"I think it's all over the place, and it's going to be all over Rio," he said. "There's just going to be a lot of stories; people you've never heard of come out of the woodwork and win a gold metal. There is no drama like the drama on the athletic field."