London — The historic first arrival by any Pope in Protestant Britain was made easier by the very event that threatened to stop it: the Falklands crisis.
To many ordinary British people, the visit of Pope John Paul II is a kind of indirect international justification of Britain's stand in the conflict, even though the Pope planned to visit Argentina June 11 and 12, immediately after leaving here.
Nonetheless, both Roman Catholic and Anglican churches held their breath as the Pope prepared to arrive, as did the British government - and underlying opposition by Anglicans and Protestants to the doctrines of the Catholic Church remains strong.
''It's a tightrope act,'' said one source familiar with the details of the visit. ''What happens if there is a great victory while he is here and dancing in the streets, and the Pope keeps on calling for immediate peace as he has been doing? The Thatcher government won't like it.
''Or what about a disaster - a big British ship sunk? Any sympathy the Pope expressed would be misread in Argentina. . . .
''The British Catholic bishops kept telling him that, if he did not come, his absence would be read as a criticism of the British war effort against Catholic Argentina. . . . It's very difficult for everyone.''
And beneath the masses and the motorcades, Britain remains the world center of the world's 65 million Anglicans.
Many here are firmly opposed to the current drive for formal unity between Canterbury and Rome, the strongest since Henry the VIII broke with Rome in the 16th century.
Now, as before the Falklands war broke out, the Pope's visit is supported by those liberal British Catholics and High-Church Anglicans who want such a formal unity. They agree with the statement of a controversial, joint Anglican-Catholic commission last March 31 that the Pope can be the ''universal primate'' of a combined church.
On the other hand, the visit is still opposed by Low-Church Anglicans and Free-Church Protestants, as well as by many Catholics, who object to any dilution of their own respective beliefs.
''I like the Pope and I like England, but I don't like the Pope coming to England,'' wrote former diplomat Connor Cruise O'Brien in April. ''England and the Pope don't go well together.''
The Falklands crisis has altered perceptions of the visit somewhat, though not of the basic Protestant-Catholic split.
''To me,'' said one source who has been close to the visit for months, ''a lot of British people see the Pope's presence as a kind of justification, an affirmation, that Britain is all right and that its actions against Argentina are legitimate.''
Somewhat more carefully, a staunch middle-aged Anglican in London said to me: ''If the Pope had not come, he would have lost a lot of goodwill in England. I think he came because he knew that and was trying to avoid it.''
A Methodist, and a stern opponent of the visit, Mrs. Edith Hill of Horley, in Surrey, said in a telephone interview, ''I have nothing against the Pope himself , but I cannot subscribe to the teachings of his church.
''I think the Pope might well gain some sympathy because he decided to come here at this time of war, but a lot of people in my church still object strongly to any idea of unity between the churches. . . .''
The Pope was said to have impressed British and Latin American bishops alike with his mass for peace in Rome. The final decision to go ahead with the visit was made in the spirit of that mass, with Latin American bishops finally dropping their objections.
''The entire visit moved to a new plane,'' one source said. ''It moved from politics to peace, and on that basis it went ahead.''
British bishops said they were relieved. Despite the misgivings of some liberal Catholic figures about the Pope's personal conservatism on moral and doctrinal issues, the 4 1/2 million Catholic worshippers here had been working for almost two years on preparations for a visit.
Immense preparations in London, Canterbury, York, Coventry, Liverpool, Manchester, Edinburgh, and Cardiff involved hundreds of thousands of people and the spending of more than $:3 million ($5.4 million).
The itinerary was revised 42 times by the Vatican and British bishops before being approved. On the first day of the visit alone - May 28 - the Pope was scheduled to go through 15 1/2 hours of meetings and events.
In the end Prime Minister Margaret Thatcher helped make up the Pope's mind by offering not to meet him, thus avoiding pictures of herself and the Pope, which would have been sharply criticized at a time of war in Latin America.
The Pope was still due at this time of writing to meet the Queen, however, who is the Supreme Governor of the Church of England, even though it is in her name that British troops have fought to regain the Falkland Islands from a Catholic country.
Before the Falklands crisis the visit was being seen as an early test of British Anglican, Protestant, and Catholic sentiment toward the kind of formal union envisaged by the joint-commission report, which was 11 years in the making.
Since the report was issued, it has been criticized heavily by Low-and Free-Church Anglicans and Protestants, and by the Vatican's office on doctrine as well. It is generally realized that unity is still a distant prospect. The Anglican church will not take any definite moves until the next Lambeth Conference in 1988.
Nonetheless, Anglican and Protestant opponents of unity have been actively stating their objections, and using the Pope's visit as an opportunity. The Protestant Reformation Society has issued a pamphlet listing 10 reasons why the Pope should not have come.