The Sacrifice (1986) on Netflix

It is Alexander's birthday, and his family have gathered at his remote cottage to celebrate. The celebrations are marred, however, first by conventional family intrigue, then by the terrifying news that the low-flying jet planes signalled the start of a nuclear war between the superpowers. As the family attempts to cope, Alexander is troubled by extraordinary dreams, and then informed by the unfathomable philosopher-postman, Otto, that he has the power to redeem the world, including his dear young son, if only he will visit the solitary Maria, who Otto insists is a witch.