Both men recently canceled their tours to focus on their health.
Bieber and Mendes were photographed arriving at Churchome in Beverly Hills, California, on Wednesday night, with Bieber behind the wheel and Mendes riding shotgun.
They kept things casual for the outing, with Mendes wearing a neutral sweater and Bieber sporting a gray hoodie and bright pink beanie.
The men have a bit of a past, as Mendes dated Bieber's wife, Hailey Bieber, before they were married. They even attended the 2018 Met Gala together, which came one month before the Biebers rekindled their romance.
While the men were at church on Wednesday, Hailey Bieber was at the Tiffany & Co. launch of the Lock Collection at the Sunset Tower Hotel in Los Angeles alongside Kim Kardashian.
In June, Justin Bieber canceled the remainder of his U.S. shows following his Ramsay Hunt syndrome diagnosis. While he returned to the stage for six shows on the European leg of his tour, Bieber said in September that he was postponing the rest of the shows after realizing the "real toll" performing took on his body.
"The exhaustion overtook me and I realized that I need to make my health the priority right now," he wrote in part. "So I'm going to take a break from touring for the time being. I'm going to be OK, but I need time to rest and get better."
The next month, Bieber officially postponed the remainder of his tour.
As for Mendes, he first announced that he was postponing some of his concerts in July, writing in part that he'd "hit a breaking point" and needed to step away to "take care of myself and my mental health."
Weeks later, Mendes canceled the remainder of his tour.
"After speaking more with my team and working with an incredible group of health professionals," Mendes wrote in part, "it has become more clear that I need to take the time I’ve never taken personally, to ground myself and come back stronger."
When ET spoke with the singer earlier this month, Mendes explained of canceling his tour, "I think I needed some time just to ground and spend some time with family and friends and gain some perspective of how beautiful it is what I get to do for a living."
Watch the video below for more on Mendes.