>How can someone reject Christianity when they haven't done the due diligence of looking at __primary sources__ for the Christian position?
Intellectual laziness. They just repeat what they see on reddit or sound-bytes they've heard from others who have no idea what they're talking about.
If they want to know what Christianity truly teaches have them read St. John Chrysostom, St. Basil, St. Thomas Aquinas, St. Bonaventure, St. Augustine, The Council of Nicaea, The Council of Jerusalem, The Council of Trent, Cornelius a Lapide commentary, Haydock Commentary, the Catena Aurea, the Latin and Greek fathers.
It is amazing how incredibly prideful and intellectually dishonest people are. If they took even 20-30 hrs of historical and theological study of what the Church always believed before the Protestant revolt, they would quickly realize that they know next to nothing about Christianity or History in general.