Sikhism arose as a religion in the Punjab region of India through the teachings of Nanak (1469-1538 A.D.). Nanak sought to synthesize the teachings of Islam and Hinduism. His teachings affirm that there is one God, and also that humans are subject to karma and go through cycles of reincarnation. As a synthesis it does not affirm important aspects of either Islam or Hinduism and so is rejected by both.
What particularly interests me is the idea of theistic reincarnation. Reincarnation is often said to be a positive view of the afterlife because people are given many chances to get it right and this is more fair than only have one life. However, if "getting it right" is so difficult that it requires many lives then it is not fair to be made to suffer to attain what is not possible in this life. On the other hand, if it is not so difficult as to require many lives, then reincarnation is not necessary.
This raises a problem for the idea of a synthesis religion. In such a religion, what is affirmed as the highest goal in life? Why work to synthesize as opposed to simply state that the religion is a new religion? Can God and reincarnation be put together consistently? If God is perfectly fair, then it is not fair to make people suffer through many lives to attain what is not attainable in those lives, nor is it fair to allow many lives when some people have done what is right while other have rejected what is right.