The Sacrament of Baptism is often called “The door of the Church” or the “Gateway Sacrament” 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, as it directly responds to Jesus’ command to “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and the Holy Spirit” (Matthew 28:19).
Accordingly, “Holy Baptism is the basis of the whole Christian life, the gateway to life in the Spirit (vitae spiritualis ianua), and the door, which gives access to the other sacraments.
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.