A devout Catholic, his work was far from finished when he was hit by a tram and killed in June 1926, aged 73, having devoted 43 years of his life to the basilica.

Dubbed “God’s architect” for his devotion, he designed some of Spain’s most famous religious sites and tourist attractions, and was put on the path to sainthood when Leo’s predecessor Pope Francis declared him “venerable” in recognition of his “heroic virtues” last year.

“Confirmation that faith inspired Gaudí in his works is found in his creations themselves, and not only in his religious works,” José Manuel Almuzara, the president of the Association for the Beatification of Antoni Gaudí, told NBC News .

“It is enough to raise one’s gaze, to have the capacity to observe, and the architecture reveals to you a man of faith,” he said.

He added that there is “a miracle that is currently being studied and we hope he will be beatified this year,” on the centenary of Gaudí.

Leo’s presence is a “tribute to the architect’s spiritual and artistic legacy, in the same temple he dedicated most of his life to,” according to the Sagrada Familia’s website.

The mass will also feature “musicians and choirs from around Catalonia to fill the Basilica with their songs,” the official schedule says.

On his first significant European trip since being elected pontiff last year and the first papal trip to the predominantly Catholic nation in 15 years, Leo has warned that escalating conflicts have pushed the world into a profound crisis.

On Monday, he became the first pontiff to address the Spanish parliament, telling lawmakers that a lack of help for the world’s migrants was challenging “the ethical foundation of the international order.”

He spoke almost exclusively in Spanish, as he did at a vast outdoor Mass in Madrid on Sunday that organizers said was attended by some 1.5 million people.

Leo will also speak Spanish at the Sagrada Familia, although he is expected to make some comments in Catalan, the predominant language in Catalonia.