The cathedral of St. Peter is not only the biggest of all Roman Catholics churches, as the Mother Church of the Catholic community it is the most famous and most visited church in the world. The present building is a Renaissance and Baroque structure, but this was built over an earlier structure that the Emperor Constantine had erected (from about A.D. 319) over the grave of the Apostle St. Peter as his memorial.
The choice of this site and the inclusion of the grave in the church, not only necessitated orientating the building westward, it also involved filling in the old necropolis and erecting high supporting walls to create a broad platform to act as a foundation. On this platform was built a basilica consisting of a central nave and four aisles, richly adorned with mosaics, frescoes and memorials, and with a a large colonnaded atrium before it. Often restored and altered, Constantine's building (known as Old St. Peter's) survived until the beginning of 16th century.
During the exile of the popes in Avignon (1309-1377), the building had deteriorated and much of its magnificence and grandeur were lost. The desire for a Mother Church of appropriate size and grandeur, together with the removal of the papal residence to the Vatican, gave rise to he first plans for a new building, and under Nicholas V (pontificate 1447-1455) work started on a new choir and transept, but soon had to be abandoned because of a lack of money. It was pope Julius II (1503 – 1513) who finally decided to pull down old St. Peter's, and on April 18, 1506 commissioned Bramante to design a new one.
Bramante's design were for a centrally planned building with a dome placed at the center of a Greek cross. This form corresponded to the ideals of the Renaissance and took up the idea of an antique mausoleum. But well over a century was to pass before the main body of St Peter's was completed in its present form, with many celebrated architects involved in the development. Bramante was finally followed by Raphael, Fra Giocondo, Giuliano da Sangallo, Baldassarre Peruzzi and Antonio da Sangallo before Pope Paul III (1534-1549)entrusted the direction of the work to Michelangelo Buonarroti in 1546.
Michelangelo's interest lay above all in the grandiose dome, the huge drum of which was completed by his death in 1564. Vignola, Pirro Ligorio and Giacomo della Porta continued his work. But changes in the liturgy introduced by the Council of Trent made yet more changes necessary under Pope Paul V (1605-1621).He commissioned Carlo Maderno to extend the existing centrally planned building eastwards by adding a nave, creating a Latin Cross; he completed the celebrating facade in 1614. Pope urban VIII dedicated the new church on November 18 1626, exactly 1300 years to the day after the first basilica had been dedicated.
In 1629, Gian Lorenzo Bernini, who was now the principal architect of St. Peter's, began to construct bell towers on the façade, but building had to stop when one of the towers collapsed as a result of a weakness in the structure. Thirty years later Bernini was to redesign the St. Peter's square and change aspect of Michelangelo's imposing dome, finally blending the whole into a uniform ensemble. The work on St. Peter's cathedral was finally finished in the 18thcentury, when the sacristy was added on the south side under the direction of Pope Pius VI (1775-1799).