In June 2015, the Vatican released Laudato Si’, Pope Francis encyclical focused on environmental issues. Pope Francis acted as an advocate for a binding agreement on climate change at the United Nation (UN) Convention conferences of the parties 2015 (COP21) citing the scientific consensus on the existence and human causes of climate change. This call for action by the Pope raised the question on the role of religion in the political sphere considering whether a religious authority could influence political matters. By building on the analytical framework drawn from the work of Corell and Betsill (2008), on assessing the influence of NGO in international environmental negotiations, this thesis developed an analytical framework for accessing the influence of Religious non-governmental organizations (RNGO) to examine the influence of RNGO on COP21. The findings also shed light on the secularization debate in International Relations.