A319 First Officer reported a diversion due to a damaged main entry door. The ground damage went unnoticed at departure airport.

Narrative

While operating ZZZ-ZZZ1; we received a call from the Aft Flight Attendant. I assumed controls and radios while the Captain took the call. The Flight Attendant described a loud noise coming from door 2L and said that she could see daylight coming from the lower left corner of the door. The Captain contacted Maintenance Control and we decided to divert to ZZZ2 to not put the aircraft and passengers at risk traveling through weather and over mountains. Once on the Ground Maintenance described the damaged door as a 2 x 4 corner bend in the door. We decided to contact operations and organize a ferry flight to come and take the passengers from ZZZ2 to ZZZ1. Because I was Relief Pilot dead-headed into the operating flight I was late to the plane. By the time I got to the aircraft all the passengers and crew were ready to go. I did not conduct the walk around myself and the Captain said that the aircraft was airworthy per his walk around. Because I was late to the aircraft I was unaware if the aircraft had been catered in ZZZ and assumed that the aircraft was airworthy.