The Orange Order in Ireland has said it cannot welcome or agree with the visit of Pope Benedict XVI to Great Britain for "biblical reasons".
Pope Benedict XVI is due to visit England, Wales and Scotland in September 2010.
The Grand Orange Lodge called on UK citizens, especially its members, to demonstrate against the visit.
Sinn Fein have accused the Order of being "firmly in the camp of the far right" by its stance.
The trip, which does not include Northern Ireland, is the first official state visit by a Pontiff to the UK.
The Orange Order is a Protestant organisation which regards itself as defending civil and religious liberties of Protestants and seeks to uphold the rule and ascendancy of a Protestant monarch in the United Kingdom.
It is best known for its parades in Northern Ireland and many senior unionist politicians are members.
"We take this opportunity to call on all the people of our land to examine the Scriptures of the Old and New Testaments, especially on the matter of eternal salvation," the Order said in a statement.
"To see that the teaching of the Pope and the Roman Catholic Church is at total variance with the Biblical message that salvation is by grace alone, through faith alone, in Christ alone.
"At the same time, we call on all members of the Loyal Orange Institution to refrain from any uncharitable acts or sentiments against our Roman Catholic fellow countrymen."
The Order said it recognised the right of the Pope to visit his flock as it recognises the civil and religious rights of all.
But it continued, "anyone welcoming Pope Benedict is in danger of appearing to acknowledge his primacy and universal supremacy as Vicar of Christ on earth".
Sinn Fein's Daithi McKay said that the Orange Order was simply an "anti-Catholic organisation".
"They are now firmly in the camp of the British far right in organising opposition to this visit. The mask has clearly slipped and the recent public relations efforts of the Order now lie in tatters," he said.
"At a time when politicians and others are trying to put together a better way of dealing with contentious Orange parades this statement is extremely unhelpful."
Grand Chaplain Reverend Alistair Smyth, a clergyman in County Down, said they did not want protests of a violent nature taking place.
He said the final decision was up to the Grand Orange Lodge of Scotland and Grand Orange Lodge of England, but that leafletting and letter writing were possibilities.
He said that members might consider rallies outlining the differences between the Catholic Church and the principles of the Protestant reformation Orange Order members espouse.
He told RTE the Order's statement was not sectarian, but he said members wanted to flag up the principles of the reformed faith.