This research is aimed at investigating Christians’ conversion to Islam and their criticism of Christianity. It is also to present their efforts in that field and to show the importance in calling people to Islam. In particular, the works of Ali bin Rabbn Al-Tabari and Ibrahim Khalil Ahmad. Will be evaluated and compared particularly the two scholars' accomplishments. These are framed in the context of the body of work in comparative religions. To achieve these objectives, appropriate research methods were utilized. The descriptive analytical method was used to form the bibliography of the two scholars and the main factors that influenced their characters. Inductive method was used to combine their efforts and to assess them in a proper way. under the right titles. The deductive method was used to deduct their approach of criticizing Christianity based on their available writings. The Comparative method was also used to compare the two approaches in order to know the main factors in each approach and the similarities and differences between them. the study found the following: firstly, there are many reasons for Christians revert to Islam and the main reason is Allah’s will, and the writings occupy a very high position as they provide new information for Muslim scholars not known before. Secondly, the efforts of Al-Tabari and Khalil are very important for those who work in the field of comparative religions, especially, in the rejection of trinity, and the texts talk about Mohammad (Peace Be Upon Him).And his prophethood.