The Sands of Time by Sidney Sheldon

Four nuns find themselves suddenly thrust into a hostile world they long ago abandoned for the safety of the convent. Unwittingly they become pawns in a battle between the charismatic Jaime Miró, leader of the outlawed Basque nationalists, and the ruthless Colonel Ramón Acoca of the Spanish Army.
Four women - and the men they are forbidden to love.
Megan - the orphan, who feels an overpowering attraction to Jaime.
Lucia - the fiery Sicilian beauty on the run for murder - and Rubio Arzano, the freedom fighter who risks his life to save her.
Teresa - whose guilty conscience finally drives her to betray her friends.
Graciela - who bears a terrible secret that almost destroys her - and the courageous Ricardo who loves her.

From the same author of the books "The Other Side of Midnight", "Rage of Angels", "Bloodline" and "Windmills of the Gods". The book is set in Spain and centres around four nuns and their unwitting involvement with outlawed Basque nationalists and the Spanish Army.