Short answers, pulled from the story.
Gabriel Marcel was a French philosopher, playwright, and music critic who lived from 1889 to 1973. He is known as a leading Christian existentialist and the author of more than a dozen philosophical books and at least thirty plays, with major works including The Mystery of Being and Man Against Mass Society.
Marcel dissociated himself from existentialism because he did not want to be grouped with Jean-Paul Sartre, despite having influenced Sartre through his weekly Paris philosophy discussion group. Marcel preferred the terms philosophy of existence or neo-Socrateanism to describe his own thought, aligning himself more with Søren Kierkegaard.
Marcel converted to Catholicism in 1929. He had been raised by an agnostic father and was not a member of any organised religion before that conversion. He was also actively opposed to anti-Semitism and supported outreach to non-Catholics.
For Marcel, a problem is external to the person examining it and can be solved with the right technique, whereas a mystery involves the person asking and cannot be stepped outside of. He argued that technology and scientific egoism wrongly reduce all human experience to problems, replacing the mystery of being with technical solutions.
Marcel influenced Jean Wahl, Paul Ricœur, Emmanuel Levinas, and Jean-Paul Sartre through his weekly Paris philosophy discussion group. He also influenced Karol Wojtyla, later Pope John Paul II, who drew on Marcel's distinction between being and having in his critique of technological change.
Marcel delivered the Gifford Lectures at the University of Aberdeen between 1949 and 1950. They were published in 1951 as The Mystery of Being, a two-volume work that is among his best-known philosophical texts.