Change of Habit (1970)

Dr. John Carpenter takes the job of running a health center in a low-income district. He enlists three women to help out who — unbeknownst to him — are actually nuns in street clothes. The church wants to improve the neighborhood but fears that nuns in full habit would not be well received. Unaware of her unavailability, John falls for Sister Michelle, serenading her with his guitar — which, luckily for him, effectively wears away at her religious resolve.