After the Decree of Milan on secularism in 313 and especially after the decree of Theodosius in 380, which established Christianity as the official religion of the Roman Empire, a completely new pastoral and liturgical situation was created in the great ecclesiastical centers of the East and the West. The mass admission of the pagans to Christianity created new pastoral needs for the baptismal process, which until then, as evidenced by the manuscript sources of the first 4 Christian centuries, only included pre-baptismal catechesis. Catechumeni (those who were preparing for Baptism) followed a program of one to three years of participation in "pre-baptismal" Catechisms, as they were called, culminating in the last 40 days before baptism. During these catechisms, the Catechumeni and Fotizomeni (enlightened) were taught everything about the Christian faith and prepared to receive the Holy Mysteries. The subjects, however, related to the content, the meaning of the Mysteries, and the Worship of the Church, were taught immediately after their baptism, with the related Catechisms being called "Mystagogic", i.e. catechisms that led the newly enlightened to the deeper aspects of their Christian existence. The purpose of the present study is to deepen through the baptismal sources of the region of Palestine the content and the liturgical dimension of the mystagogic catechisms, which were carried out in the Church of Jerusalem, during the period, which saw a remarkable flourishing and prosperity, in the 4th and 5th centuries.