The discussion about the appropriateness of celebrating Onam in the church prompted me to think more deeply about the aspects involved. Onam is the most popular festival in Kerala. While it is essentially a harvest festival, the underlying mythology of Onam is a distinctly Hindu one, about the benevolent daitya King of Kerala named Mahabali and Vamana (who is considered to be the fifth avatar of Vishnu). However, the complex* (even secular) nature of the narrative is exemplified by the fact that it is Mahabali and not Vamana who is welcomed back to Kerala on the Onam day. Onam is celebrated with joy and enthusiasm by Keralites of all communities as a cultural festival. Celebrating Onam in the Church is also an accurate reflection of the cultural identity of the St. Thomas Christians (also known as Syrian Christians) in Kerala, the community that I belong to.
The population in Kerala is about 55% Hindu, 25% Muslim and 20% Christian. When I grew up in Kerala, my friends were from all the three communities and I used to visit their homes. I used to feel more 'at home' when I was in the homes of my Hindu friends as compared to that of my Muslim friends. I was not sure why. It was surprising as Christianity and Islam are monotheistic religions (as opposed to Hinduism which is polytheistic) and they even worship the same God (the God of Abraham/Ibrahim - remember, Christianity and Islam are called 'Abrahamic religions', along with Judaism). Then it came to me in a flash - I am culturally Hindu, while being Christian by faith. This is very much aligned to the way of life of St. Thomas Christians (Syrian Christians) in Kerala.
Christianity hasn't been perceived to be a 'foreign religion' in Kerala. Kerala has had the advantage of receiving Christianity very early(from AD 52, with the arrival of the apostle St. Thomas). Another great blessing was that Kerala received Christianity from its original source (the Middle East) without getting westernized. The westernization of Christianity (most visible in the westernized images of Jesus and some of the rituals that are more reflective of the western culture than of anything fundamental to Christianity) is so impactful that many people don't realize that the historical Jesus was an Asian and that Christianity originated in Asia. Anyway, Saint Thomas Christians have lived in harmony and in close interaction with their Hindu brothers in Kerala for the last 2000 years.
This has led to the current situation where most of the Christians in Kerala practice a way of Christianity that is very much aligned to the local ('Hindu') culture. For example, 'thalimala' (the wedding necklace) used in the weddings is very similar to the mangalsutra used by Hindus (with a cross inscribed, of course). Also, many of the churches have lamps that are very similar those used in Hindu temples (with a cross added) and the bishops of some of the churches have saffron robes (similar to that used by Hindu spiritual leaders). Again, many of the traditional St. Thomas Christian names are 'indigenized' versions of Aramaic/Syriac names. For example, my surname 'Kurian' is derived from the Aramaic name 'Quriaquos' which means 'Of the Lord'. To be more precise, 'Quriaquos' became 'Kuriakose' (another popular surname among St. Thomas Christians in Kerala) and then its shortened version of 'Kurian'.
Syrian Christians show a high degree of respect for other faiths. Many of the bedtime stories told to the children in Christian households are from the Hindu puranas (Hindu mythology), and, to me, this facilitates cultural integration more than anything else. (Please see here for an amazing example of respect for other traditions- Catholic priests arranging a Brahmin priest for a funeral and participating in the ceremony!)
Of course, the Christians need to be careful not to bring in aspects like the caste system in Hinduism into the Christian community. This is a real problem as many of the Syrian Christian families in Kerala have the habit of maintaining a kudumba charithram (family history) that traces their origins to a Namboothiri ('high caste' Brahmin) household that 'adopted' Christianity based on the interaction with Saint Thomas himself. They consider themselves superior to the Christians (mostly from 'lower' castes) who were 'converted' to Christianity by the British missionaries in the 19th and 20th centuries. Therefore, the word 'Syrian' has acquired a caste dimension through it was originally meant to signify the allegiance to the Church in Syria, which was mentoring the Christian Church in Kerala. Of course, there is no way of validating the accuracy of the kudumba charithram (family history) mentioned above. However, this 'smuggles in' caste system into a religion that doesn't support any sort of caste system!
So what does all this mean? To me, 'being culturally Hindu while being Christian by faith' seems to be sustainable model. Seen in this light, celebrating Onam in the church is very much appropriate (and it is definitely not a case of 'appropriating a Hindu festival'). Also, it is not just a matter of 'eat, drink and be merry'! Celebrating festivals together is a great way to bring the community together and to enhance religious harmony. Flowers, colors, dance forms and feasts are all part of the Christian way of life. Festivals are an important part of culture, and, being part of a country culturally goes much beyond just celebrating Independence Day, Republic Day etc. If one is worried about the possible impact on 'religious purity', it must be kept in mind that many of the 'mainstream'(western?!) Christian festivals/customs probably have 'pagan inspirations' if not 'pagan roots'!
*Note: The beauty of Hindu mythology is that it supports multiple interpretations and even multiple truths (as opposed to clear black and while definitions of right and wrong). Even in the case of the Onam myth there is no clear oppressor or victim. See here for more details.