Why would a Christian/Hindu syncretic religion develop in a TL without Islam. Such a religion didnt develop in OTL, why would it develop in this TL?
If Islam doesn't exist, then it will not be able to influence the religious identity of India nor block other faiths from contributing their own viewpoints. Since Christianity was, before Islam, fairly important in Arabia, Ethiopia, and Mesopotamia, and since the absence of Islam would probably allow Christianity to grow or at least survive in those areas more than it did in reality, and since those areas had extensive contact with India, it is not implausible that there would be more Christian influence on India than there was IOTL, especially if mostly Christian nomads conquered the north in an analogue to the Ghazvanid or Mughal Empires.
That being said, I don't see the development of a Christian/Hindu syncretic religion as being particularly likely. It is pretty clear that IOTL if any such syncretic faith was going to develop, though, it was going to be from Islam, which it was. The St. Thomas Christians were too marginal and the Catholics and Protestants of later eras too distant to spur syncretization. Only Muslims were in a position of influence and prominence long enough for something like Sikhism to emerge, though who knows what might happen in the future?
I agree, the differences between monotheistic Christianity and polytheistic Hinduism are too great to allow such.
Islam is
much more monotheistic than Christianity, and yet Sikhism, which is a syncretic religion born out of Hinduism and Islam, still exists. There are similarly syncretic faiths, albeit more minor, combining Christianity and Christian influences with a wide range of local faiths. Why should Christianity and Hinduism be different?