Rerum Novarum has no direct connection with Vatican I. It is an encyclical published about a decade later. It was not a response to "Modernism," and "Modernism" is not directly related to Marxism. Modernism was a theological movement, responded to by Pius X in his encyclical Pascendi and other documents and disciplinary measures. is not an encyclical, and was not written by Paul VI. It is a Constitution (i.e., one of the major teaching documents) composed at the Second Vatican Council. Etc.