Sergio Obeso Rivera was born in Xalapa, Mexico on 31 October 1931. His father was a native of Asturias, Spain, and his mother was from Las Vigas de Ramírez, where he was raised. He entered the seminary in Xalapa on 23 January 1944 and was ordained in Rome on 31 October 1954 and while earning his doctorate in theology at the Pontifical Gregorian University. Until 1971 he held various positions at the Xalapa seminary, including prefect of philosophy, spiritual director, and rector.
