It is common to hear from Christians that convert to Islam that their motivation for the conversion stems from disbelief in a Trinity and the concept of one God as presented in Christianity. The merits of believing in a Trinity or not believing in a Trinity are not to be argued here. An aside and personal opinion is that how a person acts is far more important than what they believe. Faith by itself is not a determiner of righteousness.
There are older religions than Islam that believe in one God, and in which the Trinity is not part of the religion. This includes Judaism, Sikhism, and other religions. Unitarianism, considered a form of Christianity, does not believe in the divinity of Jesus, and sees God as one without the Trinity.
Why then do many Christians that reject the Trinity turn to Islam instead of Unitarianism, Judaism or another religion where there is belief in one God and not in the Trinity?
Islam is far larger than Unitarianism and all other religions except Christianity, so it likely that a Christian rejecting the Trinity hears about Islam and not about Unitarianism or other religions that believe in one God but not the Trinity. Also, Islam, like Christianity, proselytizes and seeks converts. Religions that do not proselytize are far less visible than religions that do proselytize.
The large size and domination by a few mega religions often drown out or dismiss the legitimacy of other beliefs and faiths.
The decision to change faiths and convert is a deeply personal one. It is important to question everything, and not be swayed by sugar coated versions of a religion or of a religion's holy books. Leaving Christianity because the Trinity does not make sense should not be an automatic jump to another religion on the assumption that it holds a monopoly on the concept of one God.
