Ubayd Allah Ibn Jahsh
This is part of historical narrative written in ibne hisham book. The truth of the story is as good as your confidence in the author. As far as leaving islam is considered, it was his personal choice. Why he did it, only God knows. Quran and logic is the criteria for judging truth. What a person does is not relevent. For establishing truth for christianity, their book and logic will be used.