A metalworker by trade, Arín became a union leader in the Confederación Nacional del Trabajo (CNT), within which he formed part of the moderate faction.

He became known to the Argentine government as a "dangerous foreigner" and, on 2 March 1912, he was prosecuted for an article he co-wrote for La Protesta in which he insulted President Roque Sáenz Peña.

[2] By 1923, Arín had been blacklisted from jobs in the metalworking industry; he was forced to change professions, managing to find work as a foreman, unloading freshly-caught fish at the docks.

After the collapse of the dictatorship, he became editor of the group's weekly magazine Acción,[4] which published its first issue on 15 February 1930, and was tasked with reorganising the CNT after years in clandestinity.

[5] From this position, he acted as the CNT's official representative in meetings with republican revolutionary committees, which aimed to overthrow the Spanish monarchy, but he came out opposed to the use of any conspiratorial methods.

[4] Instead, in January 1931, he went to La Model prison in Barcelona, where he signed a document calling for the CNT to carry out a public campaign for political freedom.

He began to campaign openly for the legalisation of the CNT, even sending a delegation to meet with the prime minister Juan Bautista Aznar-Cabañas at the end of February.

[10] Drafted by Pestaña and rewritten by Progreso Alfarache, Agustín Gibanel and Ricardo Fornells, the manifesto criticised the FAI's revolutionary aspirations as "naive", declaring that revolution could only be accomplished by the co-ordinated will of the working masses, not by a militant minority.

[11] Arín and his fellow treintistas were subsequently denounced by members of the FAI, including Buenaventura Durruti and Joan Garcia i Oliver, the latter of whom accused them of "dictatorial syndicalism".

[1] At the outbreak of the Spanish Civil War, he was in the middle of a propaganda tour; he found himself in the province of Seville when it was captured by Nationalists during their coup d'état.