The advent of Christianity in North-East India, primarily through missionary activities starting in the 19th century, has had a profound and multifaceted impact on the Scheduled Tribe societies of the region. This impact has been both transformative and disruptive, leading to significant changes in their social, cultural, economic, and political lives.

Positive Impacts:

1. Education and Literacy: Missionaries were pioneers in establishing schools, introducing formal education, and developing scripts for many unwritten tribal languages. This led to a dramatic increase in literacy rates, opening avenues for modern knowledge, employment, and upward social mobility.
2. Healthcare and Sanitation: Christian missions set up hospitals, clinics, and introduced modern medical practices, significantly improving health outcomes and combating prevalent diseases. They also promoted better hygiene and sanitation practices.
3. Social Reform: Christianity often challenged and helped eradicate certain traditional practices deemed harmful or oppressive, such as headhunting, slavery, rigid social hierarchies, and certain animistic rituals. It promoted egalitarian values and a sense of community.
4. Language and Literature Development: Missionaries played a crucial role in codifying tribal languages, translating religious texts, and creating the first written literature in many indigenous tongues, thereby preserving and promoting linguistic heritage.
5. Political Awakening and Identity Formation: The shared Christian identity often fostered a sense of unity among diverse tribal groups, transcending traditional clan or village loyalties. This contributed to the emergence of a broader tribal consciousness and played a role in political mobilization for greater autonomy and rights.
6. Economic Opportunities: Education and new skills provided by missions often led to new economic opportunities, including government jobs and engagement with the market economy.

Negative Impacts:

1. Loss of Traditional Culture and Religion: Conversion to Christianity often led to the abandonment of indigenous religions, ancestral worship, traditional festivals, rituals, dress, music, and oral traditions. This resulted in a significant erosion of unique cultural heritage and a sense of alienation from their past.
2. Social Division and Conflict: The introduction of a new religion sometimes created cleavages within communities, dividing converts from non-converts, leading to social tensions and conflicts within families and villages.
3. Alienation from Mainstream Indian Culture: In some instances, the adoption of Westernized forms of Christianity and education led to a perceived cultural distance from the broader Indian mainstream, sometimes fostering a sense of distinctness that could be exploited for separatist tendencies.
4. Dependency: Early reliance on missionary organizations for education, healthcare, and economic support sometimes created a degree of dependency.

In conclusion, the impact of Christianity on Scheduled Tribe societies of North-East India has been a complex process of modernization, empowerment, and social change, alongside significant cultural loss and identity shifts. While it brought about undeniable benefits in terms of education, health, and social reform, it also posed challenges to the preservation of indigenous cultures and sometimes led to internal divisions. Understanding this impact requires acknowledging both the progressive and disruptive aspects of this historical transformation.