Difficult Question In The Sense That You Can Haunt Yourself To Anxiety With The Answer. Yeah The 3 Basic Things In The Catechism Apply: Grave Offense, Full Conscience Knowing It Is Wrong, & Firm Free Will To Commit. Problem Is: Who Defines Grave? On 1 Hand You Can Give Yourself An Anxiety Attack If We Tally Every Little Error We Make. There Is No List. More Perplexing–explain The Person Who Steals A Bottle Of Milk During A Hurricane Crisis After All The Power Has Gone Out & All The Infrastructure Is Destroyed Including Of Course The Grocery Store Is Kaput & The Food Is Spoiling. Go Ask God. Then There’s The Nuremburg Defense. If God Already Knows We Are So Fallible Perhaps We Should Just Lock Ourselves Up In A Windowless Room & Stay There For Eternity. Don’t Forget That Even Moses Killed A Man. Then Again Even A Mortal Sin Can Be Forgiven On The Deathbed. The Only Unforgiveable Sin Is Rejection Of The Holy Spirit & I Am Not Sure What That Exactly Means.