His mother, Doña Leona, was the cousin of Dr Jose Protacio Mercado Rizal y Alonzo Realonda. Doña Leona was born on April 9, 1849 and belonged to one of the richest families in Ilocos Sur. Denied of secondary and tertiary education, she learned and mastered the Spanish language from Rev Fr Evaristo Abaya, a Filipino parish priest of Ciudad Fernandina. She was the first Filipino poetess whose lyrical and satirical works were exhibited at the Exposición General de Filipinas in Madrid in 1887 and at the Exposición Internationale in Paris in 1889. She was included in the Enciclopédie Internationale de Oeuvres des Femmes, an international encyclopedia of women’s works edited by Madame Andiz Wolska, an accomplished French writer, in 1889. Unfortunately, the original manuscripts of her works were lost during the 1896 Philippine Revolution. In conclusion, Don Belong, as Delos Reyes came to be known, inherited his talent for writing from his mother.

When his parents separated, Delos Reyes was reared by Don Mena Pecson Crisologo, an Ilocano dramatist who translated Don Quixote into Ilocano, Don Calixtofaro de la Kota Caballero de la Luna, and wrote the zarzuela Codigo Municipal. He studied at the Vigan Seminary, now the Immaculate Concepcion School of Theology, the Regional Seminary for Northern Luzon. In 1880, he enrolled at the Colegio de San Juan de Letran for his bachelor of arts degree.

When his father, Don Elías, died in 1882, he wrote articles in Spanish for the Diario de Manila, El Comercio, La Oceanía Española, La Opinion, and La Revista Popular to augment the meager allowance that Doña Leona sent him. After a while, he became the associate editor of the Diario de Manila. Two years later, his mother died of tuberculosis.

After graduating from the Colegio de San Juan de Letran, he enrolled at the Universidad de Santo Tomas for his notary public degree. Although he graduated in 1886, he was not licensed to practice his profession because he was too young to meet the minimum age requirement. Two years later, he married Josefina Sevilla, with whom he had six children.

In 1889, he founded and edited El Ilocano, the first vernacular newspaper that sowed seeds of rebellion against the Spanish friars and civil authorities and propagated the doctrines of the Kataastaasang Kagalang-galangang Katipunan ng mga Anak ng Bayan, or the Katipunan, the secret revolutionary society founded by Andres Bonifacio y De Castro at a house in Azcarraga, now Claro M Recto, near Elcano St., in Tondo, Manila, on the night of July 7, 1892, the very night when Rizal was deported to Dapitan. He did researches in Philippine folklore and history and wrote almanacs in Spanish and Ilocano that infuriated the Spanish friars and civil authorities. He wrote articles in Spanish for La Solidaridad, a democratic fortnightly founded by Graciano Lopez-Jaena, financed by Pablo Rianzares Bautista, and supported by the Junta de la Propaganda in the Philippines.

His important works during this period included Las Islas Visayas en la Epoca de la Conquista, 1887; La Expedicion de Limahong contra Filipinas, 1888; Los Triunfos del Rosario o los Holandeses en Filipinas, 1888; and Prehistoria de Filipinas, 1889. His El Folklore Filipino, a two-volume work printed at the Imprenta de Santa Cruz and published in Manila in 1890, was devoted to Philippine folklore. His Historia de Ilocos, a two-volume work serialized in El Ilocano, printed at the Imprenta de Diario de Manila, and published in Manila in 1890, discussed Ilocano mythology, paleography, philology, and sociology.

In 1896, Delos Reyes wanted to join the Katipunan, but Don Mena advised him not to do so. However, he was jailed for inciting sedition against the Spanish friars and civil authorities and was exiled to Barcelona in 1897. In an attempt to call the attention of the King and Queen of Spain, he wrote Sensacional Memoria Sobre la Revolucion Filipina de 1896. However, its impact proved insufficient. When Josefina died, he was not allowed to attend her funeral.

Upon his release in 1898, Delos Reyes served as an adviser to the Spanish Minister of Foreign Affairs. The Spanish authorities hoped that his government position and his second marriage to Maria Angeles Lopez, with whom he had eight children, would focus his attention away from the Katipunan.

In 1900, he founded and edited two nationalist periodicals, El Defensor de Filipinas and Filipinas Ante Europa, and wrote and published two books, La Religion de Katipunan and Independencia y Revolucion, in Madrid.

When he returned to the Philippines with his Spanish family in 1901, he brought the works of Mikhail Aleksandrovich Bakunin, Friedrich Engels, Karl Marx, Pierre Joseph Proudhon, and other European socialists. He served as the Secretary of State of the Revolutionary Government of General Emilio Aguinaldo y Famy and wrote journalistic articles that attacked American colonialism and imperialism.

In 1902, he formalized the break from the Vatican and founded the Union Obrera Democratica Filipina, the first labor union in the country that spearheaded the cause of democratic unionism that would free Filipino workers and laborers from oppressive and tyrannical work conditions. That same year witnessed the first organized celebration of May 1 as Labor Day, which was enacted as a national holiday on April 8, 1908. And, to support the cause of the labor union, he founded and edited La Rendicion del Obrero. Subsequently, he was jailed for inciting sedition against the American government. After him, other labor unions and labor leaders surfaced despite harsh American policies. In 1935, President Manuel Luiz Quezon y Molina instituted a pro-labor policy through his social justice program to calm the restive working population.

The end of the Filipino-American War on July 4, 1902 did not mean the end of Filipino resistance, or in a more positive manner, the Filipino desire for liberty. The laws passed by the Philippine Commission, the legislative body in the colony until the establishment of the Philippine Assembly in 1907, could be seen as an evidence of continuing Filipino aspiration for liberty. Four of these were the Sedition Law of 1901, which forbade advocacy of independence even by peaceful means; the Brigandage Act of 1902, which classified all armed resistance as pure banditry; the Reconcentration Act of 1903, which gave legal justification for hamleting to deny support for the guerillas from the populace, and the Flag Law of 1907, which prohibited the display of the Philippine Flag and the playing of the Philippine National Anthem.

A staunch anti-friar, Delos Reyes founded the Iglesia Filipina Independiente on August 3, 1902 and proclaimed Rev Fr Gregorio Aglipay as the Supreme Bishop. In his capacity as the honorary bishop, Delos Reyes founded and edited the Boletin de la Iglesia Filipina Independiente, wrote the Biblia Filipina, Calendario Aglipayano, Catequesis, Genesis Cientifico y Moderno, Oficio Divino, and Plegarias, and translated the New Testament, the Gospels according to Saint John, Saint Luke, Saint Mark, and Saint Mathew, and the Acts of the Apostles. Thereafter, American Protestants commissioned him to translate the Holy Bible into Ilocano.

In 1912, he married his third wife Maria Lim, with whom he had 13 children, after Maria Angeles died in 1910. He was elected councilor of Manila from 1912 to 1919 and senator of the Ilocos Region from 1912 to 1928. As a politician, he is best remembered for his vigorous campaign for the complete Filipinization of all city government posts.

In addition to his numerous newspaper articles, books, and pamphlets and original and translated devotional materials for the Iglesia Filipina Independiente, his other writings included the following: Ilocanadas Cuentos Filipinos, Iloilo, 1887; Articulos Varios Sobre Etnología Historia y Costumbres de Filipinas, Manila, 1888; Tipos y Cuadros de Manila, Iloilo, 1888; Prehistoria de Filipinas, Madrid, 1889; Independencia y Revolución, Madrid, 1900; El Partido Republicano de Filipinas, Manila, 1905; La Religión Antigua de los Filipinos, Manila, 1909; Ang Singsing nang [sic] Dalagang Marmol, Manila, 1912; La Libre Razón Descubre los Grandes Absurdos del Catolicismo; Controversia Religiosa entre Aglipayanos y Católicos, Manila, 1930; Los Antiquos Régulos de Filipinas, no place and year of publication; and Teogonía Ilocana, no place and year of publication.

Among the earliest to foster the growth of Ilocano literature, Delos Reyes wrote the first short story in Ilocano, Ti Langit Ti Inanamatayo, and poems, such as Adios Rosario, Bagbaga, Da Dongguial ken Namicqui, Dua Dua a Puso, Carmen Favis, Panagsina, and Pannakaallilao. Some of his poems appear in Parnaso Ilocano, Sangcareppet a Dandaniw, and Dallang Ti Amianan.

He died on October 10, 1938.