A processional cross was among a number of items damaged during a break in at Bangor Cathedral last week.
North Wales Police described the incident as "a despicable act" with "a significant amount of damage" caused.
The break-in was discovered by members of the Diocese last 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."
Police are now eager to hear from anyone who saw any suspicious behaviour or activity in the area at the time of the break-in and I’m asking the local community to remain vigilant and report any suspicious activity immediately to police.
If you can assist this investigation please contact North Wales Police control room direct via web live chat http://www.north-wales.police.uk/contact/chat-support.aspx. Alternatively phone 101 or anonymously via Crimestoppers on 0800 555 111 and quote RC1712 5600.
For comprehensive crime prevention advice on how to secure your business visit https://www.north-wales.police.uk/advice-and-support/safer-business/safer-business-intro
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