Despite being routinely present as either an enthusiastic Freemason or an equally enthusiastic agnostic, there is absolutely no evidence to suggest that Mozart did not remain a sincere Catholic for all of his conscious life. In one of his final letters, he wrote that his faith had given him the ability to no longer fear death - "And I thank my God for having given me the good fortune of having the opportunity of recognizing in it the key to our happiness. I never lie down without thinking that perhaps the next day I might not be. And yet anyone who knows me will not be able to say that in their company I am sad or in a bad mood."
Via Tea at Trianon comes the current Pope's reflections on Mozart's Requiem, to quote: -
"Everything is in perfect harmony in Mozart, every note, every musical phrase is as it is and could not be otherwise; even those opposed are reconciled; it is called "mozart’sche Heiterkeit" (Mozart's serenity), which envelops everything, every moment. It is a gift of the Grace of God, but it is also the fruit of Mozart's lively faith that, especially in sacred music, is able to reflect the luminous response of divine love, which gives hope, even when human life is lacerated by suffering and death..."