This study begins with a search for the Christian understanding of the Holy Spirit as found in the Scriptures and in the writings of the Fathers of the Early Church. It examines the exclusive nature of the Church's understanding of the activity of the Holy Spirit and reflects upon what it means for a Christian to live a life "in the Spirit". Selected documents of the Vatican II Council are examined in order to underline the shift in the Church's understanding of the Holy Spirit from an exclusive to an inclusive view of the Holy Spirit. This study is discussed within the framework of the terms exclusivism, inclusivism and pluralism which have come into use since Vatican II. It seeks to clarify the possible and probable position that the Holy Spirit is active in other World Faiths.