The island nation topped a list of 53 states - narrowly beating Lesotho and Botswana - published by the Council for Christians and Jews (CCJ).
The study analysed statistics collected by the Pew-Templeton Global Religious Futures Project which considered factors such as how free people are to preach publicly or convert.
The risk of religious hostility faced by organisations and private individuals from sources such as terrorism and armed conflict was also considered.
The Commonwealth countries where religious freedoms were found to be most under threat were Malaysia, India and Nigeria.
The UK was ranked middle in the list.
CCJ trustee Zaki Cooper said he hopes the study will encourage Commonwealth leaders meeting in London this week to hold one another to task over shortcomings in religious freedoms.
He told Premier: "The Commonwealth doesn't have military power, it doesn't have economic power but it has soft power; the power of persuasion.
"If some of the Commonwealth members can persuade these countries to make progress where they are lacking in religious freedom, I think that would be a very good thing indeed."
Click here to listen to Premier's Alex Williams speaking with the Council of Christians and Jews' Zaki Cooper: