Kelly was an American dancer and actor, best known for his starring roles in the films Singin' in the Rain and An American in Paris. 
The following is an excerpt from the biography, Gene Kelly: A Life of Dance and Dreams (p. 58):
"Somehow, driving through Mexico, observing the poverty of the peasants in abject prayer at the churches, rich with gold and silver jewels, he was sickened by the contrast. He abhorred the role of the Church in supporting the fascist Franco against the Spanish Republic. Now, seeing it firsthand, the failure of the Church, his church, to come to grips meaningfully with the physical and spiritual needs of the poor opened his eyes to the hypocrisy of what he called "organized religion." Feeding into his consciousness were his recent meetings with simpatico contacts who had influenced him with their outspoken attitudes- Robeson, Comden, Green, and many left-wing intellectuals. On the return trip, long before they saw Pittsburgh looming in the distance, he made it plain to his father that he was now a complete agnostic. His father could tell Harriet [Gene's mother] or not, as he chose. But no longer would Gene Kelly be dominated by the wishes and preferences of his mother."