The Sacrament of Baptism is often called "The door of the Church," because it is the first of the seven sacraments not only in time (since most Catholics receive it as infants) but in priority, since the reception of the other sacraments depends on it. It is the first of the three Sacraments of Initiation, the other two being the Sacrament of Confirmation and the Sacrament of Holy Communion. Once baptized, a person becomes a member of the Church. Traditionally, the rite (or ceremony) of baptism was held outside the doors of the main part of the church, to signify this fact.
To learn more about this wonderful sacrament, please visit the link below.