MADRID — Two advertising campaigns on Madrid buses are urging people to enjoy life — but for reasons that couldn't be more different. One reads, "God does exist. Enjoy your life in Christ," while the other tells readers, "God probably doesn't exist. Stop worrying and enjoy your life."

The ads began in December, and promoters of both are happy to have sparked a debate in Spain, where faith is challenged at every turn.

The idea for the eveangelical ads started when Francisco Rubiales, a 39-year-old pastor and lawyer, learned that London buses carried an atheist advertising campaign. He resolved to respond in Madrid with an opposing message.

Rubiales' modest congregation of some 50 souls, Centro Cristiano de Reunion, designated 2,000 euros of the Sunday offering to buy the ads on two buses. "Little did we know that this anecdote would become a world campaign," chuckled Rubiales, still astonished by the media's coverage of the initiative. Since the ads' launch, he said, traffic to his church's website is up 50 percent.

Meanwhile, an association of atheists and freethinkers (called AMAL) also bought advertising space on buses in Madrid, Barcelona and Malaga.

"The Church is used to occupying the whole space," said Luis Vega, a 63-year-old aircraft engineer and the president of AMAL.

"We wanted to make ourselves visible as atheists, as non-believers in God but believers in the human being," Vega added. Like the opposing side, Vega has seen a huge increase in interest since the advertising campaign began: AMAL has been so overwhelmed with support and small donations, totaling 30,000 euros (about $39,000), that there are plans to expand the message to other towns.

The campaigns reflect the larger issue of Spain's complex relationship with religion, especially the dominant Catholicism.

Former dictator Francisco Franco's national-Catholicism doctrine was embraced by many Spaniards. Although some priests dissented, the Catholic Church generally supported Franco. When Vega was young, non-baptized children could not attend school, and churches were full on Sundays.

But democracy brought change at a vertiginous speed. Although more than three out of four Spaniards identify themselves as Catholic today — according to a 2008 survey by the Center for Sociological Research (CIS) — social and behavioral trends indicate that Catholicism in Spain is about tradition, not conviction, and is a sign of identity rather than faith.

Patron saint figures are paraded in the streets, surrounded by often inebriated fiesta-goers, and soccer players habitually cross themselves before taking to the field. When it comes to following the Church's teachings, many Spaniards aren't sticklers.

The most recent data from the National Institute of Statistics show that more than one in four babies are born out of wedlock and almost half of all weddings here are civil ceremonies. Other CIS surveys show that two-thirds of Spaniards support same-sex marriage. One in every five pregnancies here ends in abortion, according to the pro-life activist group Family Forum. And fewer than one in five Spaniards attends Mass at least once a week, the CIS survey concluded.

But the Church's presence doesn't go away so readily.

Unlike in the U.S., politicians here do not mention God in their speeches, and they are rarely photographed attending Mass. However, a cross is prominently displayed on the table where Spanish leaders are sworn into office with their hands on the Bible or Constitution. Politicians march in religious parades. The cross often hangs from walls in public centers, although a court recently ordered the cross to be removed from the walls of a public school in Valladolid.

The Spanish constitution says there is no state religion. But taxpayers can opt to give 0.7 percent of their income tax (that is, part of, not in addition to, what they have to pay in taxes) to the Catholic Church. No other religion receives this concession, which one-third of Spanish taxpayers chose to use in 2007.

Opponents criticize the public financing of the Catholic Church, but others defend it, pointing to the church's charitable work, schools and restoration of centuries-old churches and monasteries.

The Spanish government does not plan to change this public financing, and a recent visit from the Vatican's secretary of state made clear the importance the church enjoys in Spain.

Spain's king, president, vice-president and the leader of the main opposition party all met with the Vatican's representative. Despite the difficult relationship between Spain's center-left ruling party and the Spanish Catholic hierarchy over legislation regarding religious teaching in schools, abortion, fast-track divorce and same-sex marriage, President Jose Luis Rodriguez Zapatero's administration attempts to foster good relations with the Vatican.

The fact that Andalusia and Extremadura, two regions where Zapatero's party has hegemony, have the highest rate of Catholic weddings may not be trivial.