I found this quote on the Humanismo Secular Facebook site. Below is the translation.
Mr. Osio asked me to erase the phrase, but I could not please him because the history of Mexico was at stake. To assert “God does not exist”, I did not shield myself behind Don Ignacio Ramirez; I am an atheist and consider religions as a form of collective neurosis. I am not an enemy of the Catholics, in the same way as I am not an enemy of the tuberculous, the near-sighted or the paralytic; one cannot be an enemy of the sick, only their good friend to try to cure them
Quote by Diego Rivera in response to violations suffered against his freedom of expression. Taken from an interview with Rene Tirado sources, the note “Diego Rivera proposes a ‘transaction’ to the Archbishop,” the newspaper Excelsior. Mexico, June 3, 1948.
The poster image of Diego Rivera painting out his famous mural
(“Dream of a Sunday Afternoon in Alameda Park”)
This mural and other works can be found at the Museo Mural Diego Rivera in Mexico City.
See you next time.
sixpointnineme 
Interesting, but in 1956 when Rivera discovered a cancer, he changed the sentence to “Conference of San Juan de Letras”… If he was afraid of his death or just believed in a peace time with the Catholic church, nobody knows…
I’m creating a blog about Rivera’s murals in Mexico: http://errantius.wordpress.com
Yes, in fact he did change the sentence to “Conferencia de San Juan de Letrán” to appease the Mexican Catholics.
I will keep tabs on your blog. Thanks for the comment.