Described by one diplomat here as "the battle for the streets", the trend has Mr Castro worried. Noting that some Protestant churches, notably the Pentecostalists, are growing even faster than the traditional Catholic Church, the Cuban leader has eased the screws on Catholicism. Some suspect him of cutting a deal with the island's Cardinal Jaime Ortega merely to string out his survival.
The President recently closed scores of so-called casas culto (literally "cult houses" but really meaning "home-worship places"), where thousands had been worshipping in the absence of church buildings.
Orson Vila, a Pentecostal minister, was arrested at his home in the city of Camaguey while preaching to 2,000 people crammed into his patio. He was tried by one of Mr Castro's no-point-in-wasting-any-time courts the same day and sentenced to 23 months for holding an illegal gathering. The sentence was later reduced to five months, possibly because Pastor Vila was converting large numbers of prisoners in Camaguey provincial prison.
"The doors of my house are open. If you want them shut, do it yourselves," he was reported to have told state security agents who came to get him. His arrest caused an outcry among evangelical churchmen in the US, but they were not echoed by many clergymen in Cuba.
"It's a kind of political thing. They want to change the government," said Pastor Estela Hernandez of the William Carey Baptist Church in Havana. "They don't preach the word of the Lord. They preach politics. They shout 'Hallelujah' and sing in apartment buildings and annoy the neighbours. Pastor Vila showed a lack of respect for the authorities. He didn't listen to them."
Evangelical ministers denied the accusation of politicking. Mr Castro, however, paranoid with some justification after several CIA plots to kill him, is said to have been concerned by the number of American ministers visiting the casas culto.
I suggested to Pastor Hernandez that it was Cuba's economic crisis that was driving people to religion. Even the two greatest successes of the revolution, public health and education, have faltered due to the emigration of doctors and teachers and a lack of medicines, school books and even pencils as a result of the collapse of the Soviet Union and the three- decade US embargo. She disagreed.
"The history of humanity is like that. After a certain experience, people turn towards God. The youth feel the need to have something greater than themselves. I think it's the same vacuum other people find throughout the world."
Pastor Hernandez's Baptist Church, whose ministry she shares with her husband, Pastor Francisco Naranjo, has seen its congregation triple from 100 to 300 members recently and now holds four services a week to fit them in.
Thirty-three years after Mr Castro's regime encouraged bible-burning, they are back in demand, Cardinal Ortega said last week, adding that "the grandmothers kept the faith alive" after the 1959 revolution.
The Cardinal received a warm welcome when he recently visited Miami, but strongly anti-Castro Cuban exiles suggested Mr Castro was using him for his own ends. The Cardinal avoided direct criticism of the regime at that time but last week, during a visit to Rome, he was more outspoken, saying: "The government has made great mistakes. It must make amends." The hardline exiles say the Church is simply jumping on the bandwagon as Mr Castro's political future looks increasingly insecure.
In another hint at "the battle for the streets", against the backdrop of the Communist Party's declining influence, Cardinal Ortega said the Catholic Church would play a greater hands-on role in social development. He cited projects to build irrigation aqueducts in farm co-operatives and to set up specialist wards in hospitals with donations from Catholics abroad.
The rise in church-going, meanwhile, has not pushed aside the traditional santeria, the voodoo-like black magic brought by African slaves. It has survived in conjunction with, rather than in opposition to, Catholicism, although it has tended to be rejected by Protestants. "Many people who apply for visas to the United States first go to see a babalu (santeria priest) to get his blessing," said a diplomat. "And in the poorer areas of Havana, babalus still do good business in love potions."