Santiago joined the Katipunan in 1893 and was among those tasked to organize popular councils in key areas in Manila and its environs. He was elected secretary of the secret society's second Supreme Council under the presidency of Roman Basa. It was at this time that he met Marina Dizon, daughter of Katipunan founder José Dizon and member of the Katipunan's women's auxiliary. In the same year, Santiago and Marina, who was already his fiancee, witnessed the wedding of Andrés Bonifacio to Gregoria de Jesús. The wedding sponsors were Santiago's half-brother and fellow Katipunan member Restituto Javier and his wife. Santiago himself married Marina Dizon on September 16, 1894 in the church of Binondo, Manila. They would eventually have eight children. The following year, Santiago was elected secretary of the secret society but he and his half-brother Restituto Javier were accused of betraying the secret society because a priest-professor of the University of Santo Tomas got hold of a coded message of the Katipunan. Since the priest was a friend of Javier's sister, the Katipunan suspected that Santiago and Javier revealed the society's existence and the two were expelled although they were never proven guilty. The Katipunan would not be uncovered until August 1896 and neither Santiago nor Javier had anything to do with it. Nonetheless, the half-brothers were among those who were arrested by the Spanish authorities and detained for months. Santiago's father-in-law was also arrested and executed. Javier was later banished to the Canary Islands. Santiago, however, was released on September 11, 1897. But he remained in contact with revolutionary forces and was appointed representative of Nueva Ecija in the Malolos Congress in 1898. When the Americans took Manila, Santiago and his wife tried to live normal lives in Meycauayan, Bulacan but had to flee to Tarlac in the course of the Philippine–American War. The couple tried to settle down in Manila anew but Santiago was later exposed as an insurgent and he had to flee to Hong Kong. When peace restored in the Philippines, Santiago returned and lived a normal life as an accountant and auditor for several companies in Manila. He died during the Japanese Occupation.
