After a thwarted contact with a clandestine Anarchist group, he and two fellow Falangists were shunned and persecuted by the mainstream SEU officials, resulting in the suicide of one of them and an ultimately ineffectual death warrant on Sacristán.
[1][2] He subsequently moved to Münster, in Westphalia (German Federal Republic) in order to study Mathematical Logic and the Philosophy of Science (1954–1956).
His political activity led him to the direction of the underground Unified Socialist Party of Catalonia (PSUC) and he had a leading role in the Catalan university movement.
He translated more than 80 works by several authors: most notably Mario Bunge, Karl Marx, Friedrich Engels, Antonio Gramsci (who may be regarded as one of his main intellectual references), Theodor W. Adorno, Karl Korsch, Lukács, Galvano Della Volpe, Galbraith, E. Fisher, Labriola, Marcuse, Ágnes Heller, G. Markus, and E. P. Thompson.
In 1975 he began the project of publishing a critical edition in Spanish of the complete works of Marx and Engels in 68 volumes, under the imprint of Editorial Grijalbo.
He was a clandestine member of the governing bodies of the PSUC and PCE, developing over many years an intense political activity, both at the university and on the cultural front.
Since the May 1968 crisis (the events of May in Paris and the Soviet invasion of Czechoslovakia), his differences with the official line of the PCC and PSUC led him to resign from nearly all his posts, though he stayed with the party until the end of the 1970s.