I came to Islam from a Christian background. I had no doubts about the existence of God, our creator, as I saw such beauty and complexity in creation that I knew it was impossible to be this way without an organized Planner, the Best of Planners, God. I also believed that we were put here to be tested, to see who would chose good over evil, etc, and to see who would worship and thank God, our creator and sustainer.
I grew up living with my mother who did not regularly attend church, but most of my other relatives did. They were of many different branches of Christianity (Methodist, Unity of God, Pentecostal, Church of God, Lutheran). Plus, my best friend was Catholic and another friend's father was a Pryspeterian minister. I attended church, Sunday school and even confirmation classes with my friends and relatives, but on a regular basis I attended a Baptist church very close to my home. I started questioning the differences between the different Christian groups at a young age. Each way claimed to be the correct way. I wasn't convinced that any one of the Christian denominations was completely superior to the others.
I met Muslims for the first time when I went to another city to attend college. I was curious to know what they believed, as I knew almost nothing about Islam. I was surprised to find out that Muslims worshipped God and that Muslims believed in all the previous Prophets like Noah and Abraham and that Muslims loved Jesus, too (peace and blessings be on all of the Prophets). Prayer, fasting, modest dress and many of the other teachings of Islam were also mentioned in the Bible, so accepting them was not a big problem for me. I found that the real difference between Islam and Christianity was in the "nature" of God and in the "divinity" of Jesus.
The Muslims claimed that God was One, wholly and completely One, separate from His creation. There is creation and there is God and they are not mixed. The Christians claimed Jesus was divine and part of the trinity. The Muslims claimed that Jesus was a prophet of God, not a "son" or part of God. The more I looked in the Bible the more the believed in the Muslims' understanding of the role of Jesus. Jesus ate food. God is not in need of any food or drink. Jesus prayed and fasted and worshipped God and why would God worship Himself?! I could find no proof of trinity in the Bible. None of the Prophets in the Bible taught the trinity. They all taught of the One-ness of God, and how could they all have been mistaken about the very basic nature of God if they were Prophets?
Also what really made the difference for me was that the Muslims had preserved their holy book, the Quran, in exactly the form it was revealed, unlike the Bible. Orthodox Muslims also had preserved the teachings of the Prophet Muhammed and they followed only what he followed and worshipped only as he worshiped, unlike the Christians groups who each worshipped in whichever way they chose. I saw orthodox Islam as an uncorrupted religion, with a true understanding of the nature of God, and endorsing a way of life that was balanced and good for the individual and the society. I was amazed at the biography of the Prophet Muhammed (peace be upon him), amazed that even his enemies recognized his virtues.
Islam is a universal religion. I felt such inner peace because Islam agreed with my innate understanding of God. That there is only One God, that His presence is apparent when one reflects on the creation, that our purpose in life is to glorify Him, that nothing other than God deserves our worship, that there will be reward or punishment in the life after death depending on what we do during our lives here, etc., etc. The more I learn about Islam the more certain I am that choosing to be Muslim was the best decision that I ever made.
I wrote in much more detail about why I became Muslim HERE, if you would like to read it.