In the wake of the Soviet occupation of Romania, he joined Oastea Domnului (the Lord's Army),[3] a spiritual renewal movement of lay volunteers as well as clerics, associated with the Romanian Orthodox Church.
In April 1957, he married Nicoleta Valeria Bruteanu (1919–96), a graduate of Bucharest Conservatory, relative of Iuliu Maniu and former political detainee.
He hosted the radio show "Lumea creștină" on Radio Free Europe, and lectured in Paris, Bordeaux, Versailles, Besançon, Dieppe, Tours, Blois, Poitiers, Nantes, Brest, Toulouse, Lyon &c. On 18 January 1996, after 27 years in exile, Sergiu Grossu returned for good to Bucharest, bringing with him the mortal remains of his wife.
In Bucharest, he founded Fundația Foștilor Deținuți Politici "Nicoleta Valeria Grossu", the publishing house "Duh și Adevăr", and the association "Centrul de cultură creștină Nicoleta Valeria Grossu."
In Chișinău Sergiu Grossu founded Centrul internațional de cultură pentru copii și tineret "Sergiu Grossu" and sponsored the creation of the Muzeul Memoriei Neamului, led by his former classmate Vadim Pirogan.