Jon had sung U2's 'I Still Haven't Found What I'm Looking For' earlier in the show, with Andrew Lloyd Webber criticising him for being more "Jesus of Las Vegas" than Jesus of Nazareth.
Jeff sang Led Zeppelin classic 'Whole Lotta Love', and was told he was "one sexy Jesus" by Dawn French.
The two were joined for the sing-off by 41-year-old Roger Wright.
After performing Jesus Christ Superstar's 'Could We Start Again, Please?', Lloyd Webber eschewed reality TV protocol by announcing without any dramatic pause that he would save Roger.
He then immediately told Jon that he had made "an incredible amount of progress" and that he would have a "fabulous, fabulous career".
Turning to Jeff, he said he had "no doubt" that the performer would one day play Jesus Christ Superstar's title role, but explained that the young man was currently "our baby Jesus".
Both contestants spoke graciously, with Jeff praising his talented opponents and the "iconic" judges, and Jon saying: "Thank you so much."
Another contestant will leave during the show on Monday, July 23 at 9pm.
Photo gallery - Superstar contestants: