In 1979, a year after being elected, the Polish Pope returned to visit his homeland, and started a revolution. Millions turned out to see him wherever he went.
No Communist country had ever seen anything like it.
Crowds were only supposed to turn out at state-sponsored rallies, in support of the system and never spontaneously.
Pope John Paul II greets Mikhail Gorbachev
John Paul II did nothing so crude as to criticise the authorities. He merely gave the people a sense of moral superiority and hope.
In his home town, Wadowice, a massive crowd packed a park to listen to him.
Krzystof Gurba, a university lecturer, later recalled the effect the Pope's words had.
"I think it was an inspiration," he said.
"It was a seed that grew later on. I'm sure it was the starting point for the next year's uprising of Solidarity."
Solidarity was the Communist bloc's first free trade union, set up in the heat of strikes led by Lech Walesa at the Lenin shipyard in Gdansk in 1980.
Inspired by their Polish Pope, they humiliated the government.
Lech Walesa signed the document recognising Solidarity with an outsize souvenir pope pen.
Polish Solidarity leader Lech Walesa pictured in 1980
Mr Walesa recalls how the Pope brought back moral values.
"They defeated the Communist system," he said.
"The Communists were as frightened of those values as the devil is of holy water, so in that sense the Pope is the author of the victory over Communism."
The truce with the Communists was fragile, and Solidarity was soon banned.
But even General Wojciech Jaruzelski, the man who imposed martial law in December 1981 to squash the movement, understood that the Pope's influence had lit a flame that could not be doused again.
"Of course," he said later, "the social and economic conditions were ripe for a protest, but the Pope and his teachings served as a detonator which liberated the spirit of society and gave it strength."
In 1987, on his third visit to Poland, the Pope prayed at the Gdansk monument to workers killed by the Communist regime.
He seemed to take a conscious step from the pulpit into the political arena.
The Pope returned to his native Poland in 2002
Celebrating mass before a crowd of about 750,000, many waving Solidarity banners, he praised their struggle.
"There is no struggle more effective than solidarity." The crowd roared its appreciation, understanding he meant the word to have a capital "S".
Now the scene was set for revolution, not just in Poland but all over eastern Europe.
The sense of inner freedom inspired by the Pope was spreading throughout the Communist bloc, and with a new Soviet leader, Mikhail Gorbachev, simultaneously loosening the chains, it was only a matter of time before the people would break free.
At the end of 1989 - the "year of revolutions" in eastern Europe - Mr Gorbachev went to the Vatican for an historic meeting, between the man who nudged Eastern Europe towards freedom and the man who let them go.
It set the seal on a year, indeed a decade, of change in the Communist bloc.
Free elections in Poland in June 1989, won overwhelmingly by Solidarity, started a chain reaction which brought down Communist governments throughout the Soviet bloc, and ended with the dismantling of the Berlin Wall in October 1989.
Doubtless, it would all have happened anyway, but without the Polish Pope it might have taken much longer.