Whenever you walk into a Catholic church, there is a physical difference between the gathering place and the nave, where the pews are located.
This physical transition is called the "threshold," and has a rich spiritual symbolism behind it.
Between two worlds
The Catechism of the Catholic Church specifically mentions the threshold in its section on the liturgy:
Finally, the church has an eschatological significance. To enter into the house of God, we must cross a threshold, which symbolizes passing from the world wounded by sin to the world of the new Life to which all men are called. The visible church is a symbol of the Father's house toward which the People of God is journeying and where the Father "will wipe every tear from their eyes." Also for this reason, the Church is the house of all God's children, open and welcoming.
One way that some churches highlight this symbolism is by emphasizing the door that separates the gathering place and the nave.
The US Bishop's document on building churches highlights the significance of the door:
The doors to the church have both practical and symbolic significance. They function as the secure, steady symbol of Christ, "the Good Shepherd and "the door through which those who follow him enter and are safe [as they] go in and go out." In construction, design, and decoration, they have the ability to remind people of Christ's presence as the Way that leads to the Father.
Likely we may never notice or think about the threshold of a church, but our mind can subtly register such things and when there is a dramatic difference between the "two worlds," our heart can better prepare itself to receive the Lord at Mass.
Everything inside a church can raise our hearts to God, even the place that we walk through to enter into the house of God.
