Every house has a door, which is necessary to access the interior and to participate in the life of the family within. In this sense, the door is “ordered” toward what happens inside; without it, it is impossible to join the family inside for dinner, even though that may be the reason for coming to the house. Baptism holds an analogous place to a door in the order of the Sacraments, all of which “are ordered to [the Eucharist] as to their end” (St. Thomas Aquinas, Summa theologiae III, 65, 3). In an earlier article, we mentioned that the nave of a church building symbolizes the “new earth”, the place of perfect communion between God and Man at the end of time. Now we can see how the baptismal font, which initiates us into that right relationship, over time has become architecturally associated with the physical door of the church. This association continues even for those of us who are already baptized; there are holy water fonts at each door of a church to remind us of the sacrament that gives us admittance to the life of grace, particularly to the Eucharist, the very “source and summit” of that life.Holy Baptism is the basis of the whole Christian life, the gateway to life in the Spirit, and the door which gives access to the other sacraments.”
Catechism of the Catholic Church, 1213