Educate yourself about the history of Catholicism’s mother church and the cathedral church of the Bishop of Rome here.

There is so much that can be said about the image of the temple in the Scriptures. We are called to be temples of the Holy Spirit, all of us are living stones built up into God’s temple. The church is a temple built on the foundation of the apostles and prophets.
 Is there a theology of architecture? Check out these pages which comment on this through reflection on an exciting new church being built in Kansas. Check it out over at Creative Minority Report.
Are there any principles for a Christian church, or can you do what you want? Is Gothic or Romanesque or modern or baroque necessarily any better or worse? I would argue that there is actually a template of sorts for worship buildings and that it is all in the Old Testament. 
After all, if the Holy Scriptures are divinely inspired and some of the Holy Scriptures include plans for a temple, then maybe we ought to pay attention. The principles are laid down when God tells the Hebrew people to build first a tabernacle for their desert wanderings and then a temple which follows the same pattern. 
The pattern is this: First, there is a large area for the people of God to gather. In front of that is another court where the dedicated servants of God offer the sacrifice. Thirdly, beyond that is the Holy of Holies. For millennia the Christian church followed this simple pattern with nave, choir (or chancel) and sanctuary. Part of this pattern is the fact that the floor plan is linear–not round. Linear indicates a Christian theology of life. We are on a journey from A to Z from Alpha to Omega, from Baptism to Heaven. Round worship spaces are pagan and reflect a pagan belief in endless cycles of being etc etc.
The second principle is that the building is a kind of sacramental space. Everything from its design to it decoration ‘incarnates’ the Christian truth. The furniture, art, objects, vestments–everything has meaning. Everything points beyond itself to some eternal truth.
Thirdly, the whole place should be beautiful. This is because it is meant to be the threshold of heaven. It is the portal into another world. It is the glimpse of beauty here which points us to the everlasting gaze into the face of the Beautiful One in glory.
Needless to say Catholic Churches of the last thirty years or so do not exhibit this. Architects and theologians designed churches that look like enormous hamburgers, flying saucers, teepees or vast ice cream cones that have fallen upside down. Carpeted and cozy and built on the cheap, they reflected a falsely egalitarian theology, a cozy, comfort zone church where the liturgy was ‘the gathering of the people of God’ and the aim of the church was to get people together to make the world a better place.
The problem is–if I’m going to join a club to make the world a better place I might just as well join the Rotary Club or the Junior League of volunteer for the Peace Corps.
A beautiful temple of God reflects the same three principles in our own lives: we should be on a journey to God. Everything in our life should be radiant with deeper meaning and our lives should be ‘something beautiful for God.’ If so, then we will be temples of the Holy Spirit and the church will be that ‘radiant church without spot or blemish’ made perfect by the blood of the Lamb, and each of us will be living stones in that temple.
UPDATE: More splendid photographs of St John Lateran over at NLM