Years ago, I worked with a wonderful woman from Poland (in Aslip IL).
She said that they pretended not to go to church, and the Police pretended not to see them there.
That’s probably the way it was in a lot of places — especially where the church was still strong and respected.
But I do remember reading about young Fr. Karol Wojtyla having to sneak out at night in street clothes to meet with people for religious activities not approved by the state.