The second sacrament of Christian Initiation for the conferment of the gift of the Holy Spirit has not been taken up in a detailed research in the bilateral dialogue between the Orthodox and the Roman Catholics in the context of the Ecumenical Movement, although it is mentioned in several documents of this dialogue. The purpose of this study is the contribution to this bilateral dialogue. It presents the historical development of the sacrament of the gift of the Spirit from the New Testament witness in Acts up to the different peculiarities in the East and the West. Also, the development of the theological interpretation of the gift of the Spirit is presented in order to reach a crucial question that is the theological significance of this gift in Christian Initiation. Many aspects of the spiritual gift are presented in the context of Christian anthropology, ChristοΙοgy, Pneumatology, Ecclesiology and Soteriology.