Complex sentence: Spreading Islam in Chad was very slow because the Arab people who were spreading religion did not enter there because of it's geography because it is rocky with mountains that people did not travel inside it, and until now it is behind all other Muslim countries.
Split into simpler sentences: Spreading Islam in Chad was very slow because the Arab people who were spreading religion cannot enter there because of it's geography. It is rocky place that people did not travel inside it, and until now it is behind all other Muslim countries.