During her room audition, the Irish panellist told Abi he wasn't sure her style of performing was right for the show.
But after singing her own song 'Jigsaw' at her arena audition, Abi was shocked to see the crowd applauding on their feet.
"I wasn't sure the first time we saw you, I was wrong." admitted Louis.
Sharon Osbourne added: "It's answered the question of whether you will translate in a big arena and you got a standing ovation."
Gary Barlow told the 19-year-old her audition was "magical".
"As everybody started to stand, it was like your dream come to life right in front of you," he added.
"I was pleased to be here for that moment, what an incredible audition that was - well done."
I just think that you are so courageous for writing this new song and sharing it with all of us today.
Nicole Scherzinger to Abi Alton
Speaking after her audition, Abi admitted that she wasn't a "conventional" X Factor contestant.
"I don't think that makes me any better or any worse," she said.
"I think I'm just a little bit different - and that is a risk for Louis to put me through. I did understand his reservations.
"Initially, it did make me think I would have to do a cover in the arena to please people - but when my original song came out of me, I thought, 'I'm not going to try to be something I'm not'. If I hadn't got through, then it just wasn't meant to be."
Meanwhile Dubliner Melanie McCabe is now one step closer to her dream of making it to the live shows after receiving four 'yes' votes from the judging panel.
Returning to the show for the fourth time, Melanie said this would be her last attempt at X Factor success.
She explained her family were losing their home before her rendition of David Guetta's 'Titanium' earned her a place at Boot Camp.
Nicole Scherzinger told her: "You not only have a great big voice, but you have this effortless control."
Melanie I don't know what went on with you on this show before, I was not here, I don't care and I think you are sensational.
Louis added: "I wanted you to be so good today and you were absolutely amazing.
"I don't know what happened last year; I think you are a star.
"And it's not because you are from Dublin," he joked.
Fellow returning hopeful Jade Richards was also put through to Boot Camp, but the judges had reservations about her arena performance.
"I think you are a fantastic singer but there is something that happens in your auditions, there's no climax to it," Gary told her.
"There are people here who don't sing as good as you who have had standing ovations today."
"When you perform you disconnect from the audience," agreed Sharon. "There's no passion, there's no fire inside you."
The X Factor is back on UTV on Saturday and Sunday at 8pm.