Sant Garmon at Abersoch on Gwynedd's Llŷn peninsula is on a list of locations that could close in north Wales.
But campaigners said on some days there was "standing room only" in the church, which mainly serves holidaymakers.
The Diocese of Wrexham said it was "regrettable" that campaigners "thought it necessary" to petition the Pope.
Earlier this year Bishop of Wrexham Peter Brignall announced plans to shut 22 of the 62 Catholic churches across north Wales by 2020.
Campaign group Friends of Sant Garmon said: "No one seems to have the slightest idea why is it being closed. It doesn't make any sort of sense."
The church opens between Easter and October with services held by priests who holiday in the village and stay in luxury accommodation attached to the church.
"The scheme works like a watch - we have a full supply of priests for each year from all over Britain and beyond - even from the USA," added campaigners.
If the church closes, worshippers would have to travel to Pwllheli, a journey campaigners claimed would be difficult for many.
They have now sent a petition to the Vatican, asking for intervention.
"We hope that Pope Francis can see it is all a terrible mistake and [will] keep it open," they said.
A spokesman for Wrexham Diocese said: "It is regrettable that the campaigners calling themselves the Friends of Sant Garmon should have thought it necessary to petition the Holy Father to rescind the proposal for the closure of the church in Abersoch.
"The bishop has initiated a programme of evangelisation to be piloted on the Llŷn in the new year.
"He acknowledges that some within the church will find the Pastoral Plan painfully challenging but its purpose will be to inspire, renew and strengthen the faith of all believers."
Campaigners are calling for the Pope to save a threatened Catholic church set in an idyllic seaside village.