There is an interesting video series on youtube by a former Christian in which he painstakingly details the reasons for his de-conversion. Its in many parts but I what I have seen of it is done very well. I especially liked this clip below in which he explains that its not just one thing that causes a Christian to de-convert. There might be one thing that was the "final straw" but there are always multiple reasons. He also shows how that when a Christian doubts one aspect of their religion, they can rely on other parts of it to continue believing. Another important fact he mentions is that de-conversion typically takes years. Its not something that happens overnight.