The official census figures are considered unreliable and an underestimate of the number of Christians in India. Some efforts toward correcting this by Christian missionaries follows:

According to the Center for the Study of Global Christianity (CSGC), a demographic research center located on the campus of Gordon Conwell Theological Seminary, South Hamilton, Massachusetts, USA, Christians account for 5% of Indian population in 2015:

Another estimate from CSGC reveal Christians numbering 4.7% of Indians in 2010 and projected to reach 4.9% in 2020:

The same institute estimates that Christians constitute 9.0% of Telugu-speaking people in 2013:

Telugu is the 3rd largest mother tongue of Hindus (67.5 million), and 9.0% of its mother tongue speakers are Christian. — Global Christianity (@CSGC) April 25, 2013

Christian Aid estimated Christians in India as numbering 71 million (5.67%) in 2013.

Projections

In the book The future of the Global Church: History, Trends and Possibilities by Patrick Johnstone (2014), the author estimates that India had 64.5 million Christians (6.19%) in 2000 and that India will become the fourth largest Christian country in the world after USA, Brazil and China with a Christian population of 159.3 million in 2050:

According to the same book, the estimate of the number of Protestants in India is 21 million (1.97%) in 2000 while it is set to become 48 million by 2050:

A color-coded map from the book illustrates the percentage distribution of Christians in South Asia today:

Credit: @sighbaboo