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Díaz was born in Paoay, Ilocos Norte. He was a member of La Liga Filipina, which José Rizal founded to peacefully promote reforms in the Spanish colonial administration of the Philippines. Being a resident of Tondo in Manila, he joined the La Liga council that was headed by Andrés Bonifacio, who founded the Katipunan.
Díaz was one of the signatories of the Pact of Biak-na-Bato in 1897. He joined the revolutionaries exiled in Hong Kong as one of the conditions set forth by the Pact.