With precedents in Great Britain and the United States, the ads run on Madrid buses, reading "There's probably no God. Now stop worrying and enjoy your life."

While defending freedom of speech, the bishops stated that "public spaces that citizens are obliged to use should not be used for public messages that offend the religious convictions of many of them."

Christians have answered the campaign in Britain and the United States with various tactics.

In England, a Christian group reported the ads to the agency that monitors advertising, basing their complaint on a policy that prohibits ads from making unfounded claims. The Christian group contended that there is no proof for the ads' declaration that God probably does not exist.

In the United States, meanwhile, where the bus campaign ads read "Why believe in a god? Just be good for goodness' sake," a Catholic mom answered with her own advertising campaign, also for buses. Her ad read: "Why believe? Because I created you and I love you, for goodness' sake" and it was signed, "God."