A processional cross and other items were damaged during a break-in at Bangor Cathedral.
North Wales Police described it as "a despicable act" with "a significant amount of damage" caused.
The break-in was discovered on Friday morning.
The Very Rev Kathy Jones, Dean of Bangor, said: "All they are really doing is putting themselves in danger and creating unnecessary and heartbreaking work for us to clear up."
A North Wales Police spokesman said: "This was a despicable act damaging such a well-loved and significant local building and every effort is being made to trace those responsible.
"It was a shock and extremely saddening to find that the cathedral had been broken into last night or early this morning and damage caused inside the building."