conversion is great sin which no god or religion will forgive
But reversion is not. Unless you (in general) are a believer of the Theory of Evolution, then you believe and know that we're all offsprings of Adam and Eve, who according to muslim beliefs were Muslims as revealed by the Holy Quran. So when a non-muslim chooses to become a muslim/muslimah, he or she is basically reverting back to Islam. Although it's termed conversion.
In order to believe that, you first have to believe that 1. we are offsprings of Adam and Eve and 2. that there exists one supreme Creator of this world and all that lies beyond this universe and 3. Have the know-about of the history of Religion (as a whole).
You, then will be able to see, how the mankind developed into different tribes, races, cultures, religions and branched off. Islam in name, came about way after Christianity and Judaism, and that is because it leads back to the straight path/the original path, before people diverted from the true religion of God.
This is my belief, and the belief of majority of muslims.