Though he’s only been at St. John’s for nine months, Jackson Erdmann knows all about the craziness that engulfs the campus when the Johnnies play St. Thomas. “It’s all anyone is talking about,” the freshman quarterback said. “There’s so much hype, I’ll probably have some butterflies. But I have to stay calm and composed.”

He’s already proven he can handle that. Going into Saturday’s home game against the fourth-ranked Tommies, Erdmann, of Rosemount, has thrown for 12 touchdowns in his first three college games. His unflappability — and an offense more willing to pass after the graduation of all-time leading rusher Sam Sura — has led St. John’s to a No. 6 ranking in the poll and made Erdmann one of the top passers in Division III.

That same serenity carried Erdmann through something far more fearsome than the Tommies’ defense. In late 2013, Erdmann developed acute cerebellar ataxia, a condition that left him unable to walk, stand or sit upright. Before his senior season, while his Rosemount teammates hit the weight room, he spent his days in a school hallway learning how to walk in a straight line again.

He recovered in time to have a brilliant senior season, one that earned him a place as a walk-on at Penn State before he transferred to St. John’s last January. Erdmann said he feels lucky, grateful and blessed to continue his football career — and the Johnnies feel the same about his choice to do so in Collegeville.

“We figured we were going to have to throw the ball a little bit more this year,” Johnnies coach Gary Fasching said. “That was predicated on the quarterback being able to get the job done. And Jackson’s come in and done that for us.

“He’s a very steady kid who doesn’t get rattled. He’s overcome a lot of things, and that says a lot about him. He’s a fighter. He comes out and competes hard. And with his composure, that’s a great combination to have at quarterback.”

Erdmann did not play last season at Penn State but left with no ill will. With great depth of talent ahead of him, he decided to transfer to a place where he could get significant playing time, and he had been impressed with St. John’s when Fasching recruited him in high school.

He doesn’t run the ball much — “I’m not fast enough,” he said with a laugh — but Erdmann has connected on 40 of 56 passes for 627 yards. He ranks second nationally in Division III in passing efficiency, is tied for second in touchdown passes and has yet to throw an interception. Though the Johnnies still run the ball about twice as often as they pass, they are averaging 235.3 passing yards per game and 233.7 on the ground.

In spring ball, Erdmann was among nine quarterbacks vying to replace Nick Martin. He won the job with a big leap forward in fall practice, as he became more familiar with the offense and with a stacked group of Johnnies receivers.

“You could see his dedication and work ethic right away,” said Zack Sundly, a running back who is among five players to catch a touchdown pass from Erdmann this season. “He’s a student of the game, and he already understands our offense completely. He’s really grown and blossomed.”

Erdmann has no lingering effects from his bout with ataxia, which kept him off the field for about five months. After recovering from mononucleosis in December 2013, he was diagnosed with the condition, caused when his white blood cells moved from fighting the mono to attacking healthy brain cells. Unable to walk, stand or sit without falling over, he spent four days in the hospital and three months in physical therapy.

“It was pretty scary,” Erdmann said. “When I was getting better, I tried playing catch with my dad, and I’d throw into the ground right in front of me. Then I tried throwing with the guys, and I’d put it into the dirt, 15 yards in front of the receivers.

“I didn’t know if I’d play football again. It was a long haul, but we got there.”

The same patience and persistence that helped Erdmann recover will be paramount against the Tommies. To win, the Johnnies must limit their mistakes and maintain poise.

That might not be easy in a rivalry that could draw as many as 17,000 fans to Clemens Stadium, but Erdmann is eager to see what he can do.

“It’s going to be crazy,” he said. “But it’s going to be really fun. I feel so blessed to be here and to be able to play this game.”