TEMPE, Ariz. -- Moments before his short tenure as the Arizona Cardinals' backup quarterback was about to come to an end, Josh Rosen started smiling.

It was Sept. 23, late in the fourth quarter of Week 3 against the Chicago Bears. The Cardinals were down 16-14 with 4:31 left. The offense had slowed to a halt yet again. An 0-3 start was on the horizon.

Arizona made the organizational-defining decision to bench Sam Bradford in favor of the rookie the team drafted five months earlier with the 10th overall pick. To that point, Rosen had yet to take an NFL snap. But he was smiling.

"Once I'm not smiling, you guys should be worried," Rosen said. "I'm a happy guy. I'm very demanding of my team, as I expect them to be of me. I'm always a positivity guy, and I will continue to be."

Rosen didn't look nervous. He tossed a football in the air to himself as the kickoff took place. He checked his wristband, which housed the playcalls. He headbutted his teammates.

He looked calm.

He looked ready.

He looked confident.

"I'm a very confident person," Rosen said. "Even if I have no reason to, I'll find a way to fool myself into going out there with full confidence that we're going to win every single game we play.

"Every time I step on the field, I expect to throw a touchdown or hand off a touchdown or lead the team to the end zone in some way. In my head, if I don't, it's a surprise, and something's wrong, and we've got to fix it so that we do next time."

Some of those who have known Rosen the longest believe his confidence was ingrained in him at birth.

"I'm very confident in my arm talent, and I think I can put the ball anywhere it needs to be." Josh Rosen

"I would have to assume a lot of it is inherent -- just part of his DNA and his makeup," said Jason Negro, Rosen's high school football coach at St. John Bosco in Los Angeles.

After all, Rosen comes from a successful family.

Both his parents are Ivy League-educated. Both were also accomplished athletes.

After his father, Charles Rosen, went to Penn, he became an orthopedic surgeon whom former President Barack Obama once considered for surgeon general of the United States. Josh's mother, Liz Lippincott, is a Princeton graduate and has worked as a book editor. She's also the great-great-granddaughter of Joseph Wharton, the industrialist who founded the Wharton School at the University of Pennsylvania, according to The New York Times.

But Josh Rosen built on his lineage and genes, forging his own confidence through sports, starting on the tennis courts.

"You're by yourself out here," Rosen said. "So you've got to be confident. Even if you're not, you at least have to play the part and try to keep pushing until something good happens, then you start to gain it and keep going. So I don't know. It's just kind of who I am. Not even football, but anything. I'm very confident in what I do. Even if I'm not, I'll act like I am."

Steve Whitehead started coaching Rosen in tennis when he was 5 years old. When Rosen was 8, he began playing in tournaments. By 12, Rosen was one of the best tennis players in his age group in Southern California, if not the country.

It was during those years that Rosen developed the mindset he has carried with him to the NFL.

"He was very aggressive," said Whitehead, who taught Rosen to attack the net. "He probably could've stayed back and been more careful, but the way he played the tennis game, I was grooming him to be an aggressive attacker."

Rosen lost his share of matches because he was overly aggressive, Whitehead told ESPN. But he won some because of that, too.

Sound familiar?

Early in Rosen's tennis career, Whitehead noticed that Rosen always believed he'd win. Whitehead saw a "fearless" boy -- who eventually turned into a fearless man.

"In tennis, there are some people that take it, and some people wait for the person to give it to them," Whitehead said. "He's a taker."

When Rosen quit tennis at 13 to pursue football, he brought his confidence with him.

It didn't take long for the Bosco coaching staff to see what they had in the skinny quarterback. When Rosen was an incoming freshman, he participated in a weekly camp during the offseason, throwing to juniors and seniors who treated Rosen like he was one of them.

"You couldn't tell he was such a young guy," Negro said.

Former offensive coordinator Chad Johnson remembered Rosen grasping Bosco's offense easily as a ninth grader. Rosen had the physical tools at a young age, but he was able to pair them with an intellect that was far above that of his peers. And the more Rosen accomplished, the more his confidence kept building.

He practiced with the varsity as a freshman. Heading into his sophomore year, he began going to national camps and playing on the 7-on-7 circuit. That's when Negro saw Rosen's confidence accelerate.

"There wasn't a throw he couldn't make," Negro said. "There wasn't a read he couldn't make."

Rosen knew he could fit a pass into a small window throwing off his back foot while falling over, Johnson said. But Johnson tried to teach Rosen that just because he could make that pass didn't mean it was the right play. Johnson translated his message into something Rosen could relate to: analytics. He broke down the risk for Rosen, stressing the need to make the high-percentage play instead of a would-be interception.

But that aggressiveness followed Rosen to UCLA -- where he threw 59 touchdowns with 26 interceptions -- and to the NFL. With the Cardinals, he has thrown six interceptions and five touchdowns, and he has the fifth-highest aggressiveness percentage, according to the NFL's Next Gen Stats, which measures the number of passes thrown with a defender within 1 yard of the receiver.

"Sometimes, you've just got to trust your receivers," Rosen said. "I think I've gotten better throughout my career at giving my receivers, I call them '60-40 balls,' where they're contested, but you try to give your guy the edge with ball placement. I think a lot of the time, I just trust a lot of the guys that I have, and they've been able to make some really good plays for me. But also, I'm very confident in my arm talent, and I think I can put the ball anywhere it needs to be."

His Cardinals teammates love that confidence.

"I know he's going to throw the ball," running back David Johnson said. "He's going to take those risks, be a gun-slinger and just give the receiver a chance to catch the ball."

Said receiver Christian Kirk, who met Rosen during high school camps and recruiting trips: "It's not like he's a cocky guy. It's not like he's out there running his mouth, but he'll let you know he can ball, and he's a competitor."

The week after Rosen was drafted, he showed up at Johnson's office at Mission Viejo to start working on the playbook. They both worked on the board, and Rosen used flash cards to learn the plays. Then he spent between six and eight hours per day committing the playbook to memory so he could report for organized team activities knowing everything.

That preparation got Rosen ready for a situation like the one at the end of Week 8's game against the 49ers. With 2:16 left and the Cardinals down five, a confident Rosen stepped into the huddle.

"We're about to win this f---ing game," he said.

And they did.