Antoine Sallis was rejected for a car loan because his credit score was 378.About a decade later, he was a millionaire.He repaired his credit and used debt to build wealth.This as-told-to essay is based on a conversation with Antoine Sallis, author of "The Great American Credit Secret: Part 2." It has been edited for length and clarity.The best thing that ever happened to my finances felt like the worst thing possible at the time. I was in my late 20s, working as a sales rep at Teen Mobile. I was proud of that job and felt like I was making moves. My next step was to get a decent car, better than the $300 cars from Craigslist that I'd driven in the past.One day, I walked by a dealership with a sign that said, "Your job is your credit." I took that to mean that as long as I had a job, I could get approved for a loan. I already knew I'd have to go somewhere lenient to get a car loan, but this seemed perfect. I didn't think I had any credit, but I definitely had a job.To make the purchase even more of a slam dunk, I saved a $500 down payment and picked the cheapest car on the lot, which cost about $9,000. I strolled in to apply, filling out the long paper application. I waited for about 20 minutes. Then, the sales rep came out, barely containing his laugh. He told me my credit score was 378 so low that I was an exception to their rule, and they couldn't give me a loan despite my job.I learned about the potential of responsible debtI walked out of there feeling dejected. I'd been planning to drive off that lot. On my bus ride home, I started thinking about credit, which I knew basically nothing about.I got tunnel vision, and started to learn everything I could about credit. I started with books and videos. Later, I called people who worked in finance, offering them $60 to answer my questions for 30 minutes.I cleaned up my own credit, which started with understanding why it was so bad. Once I pulled my credit report, I saw I had unresolved accounts I didn't even know about and student loans with late payments. I started paying my debts on time to build a positive credit history. Soon, people began asking me to help them understand theirs.As I learned more, I realized credit could be an incredible tool. Once my credit score was better, I took loans to invest in real estate. Most people think debt is bad. But I realized debt can give an opportunity: if you can take out a loan and triple your money over the life of the loan, that's a win.My mindset made me a millionaireWhen I embraced that mindset, things started changing rapidly for me. I started a credit repair company and continued to invest in real estate. By 2020, my company was doing very well, and I was a millionaire. Today I'm 42, and I'm a multimillionaire with a 780 credit score.In addition to learning about credit and debt, I've seen that your mindset can make a huge difference in your outcomes. Some people in my life are constantly looking for a handout, and they're stuck in a mindset of believing they can't do things. I used to be that way, but I changed.I became a millionaire in my mind before I ever became one at the bank, because I adopted a strong mindset. I became focused and driven.I pay for my son's college and give him an allowanceThe importance of mindset is what I want my son, who is 18, to take from my story. I can see he's learned that lesson. He used to have C's and D's on his report card, but since he moved in with me, he's a straight-A student. He's about to graduate, and I'm rewarding his hard work by paying for college.Throughout high school I gave my son a generous allowance, with the expectation that he'd keep his grades up. I pay for everything he needs, but if he wants money to hang out with friends, he knows that's his responsibility. He's seen my financial transformation, and I've talked with him since he was 10 about the importance of credit. Today, I think he knows more about credit than most people, and he even helps with my business teaching others about building their credit.Now, I have good enough credit to drive luxury carsThat day when I was rejected for a car loan, I laughed along with the salesman. But really, it was no laughing matter. Today, I have multiple luxury cars, which I love. The Lamborghini Gallardo is probably my favorite, though my Bentley is really comfortable.I want people to know that no matter how dismal your financial situation feels, you can change it. Leveraging credit and debt is a hidden avenue for people who didn't grow up wealthy to build their financial security. You can access more and more, as long as you do it responsibly.