Final answer:
The Old St. Peter's Basilica was the largest church in Europe until it was replaced by the new St. Peter's Basilica in the 17th century, reflecting the architectural and religious significance of the Catholic Church during the Renaissance.
Step-by-step explanation:
The church that was the largest in Europe until the new St. Peter's Basilica in Rome was constructed in the 17th century was the Old St. Peter's Basilica. The original church, built in the 4th century by Constantine the Great, stood as a testament to the religious and architectural ambitions of the early Christian church and the Roman Catholic tradition. Over time, Old St. Peter's Basilica served as an important pilgrimage site and the central church of the Vatican before ultimately being torn down and replaced by the new St. Peter's in the 16th century, which was part of a broader effort to reestablish and visually embellish the power of the Catholic Church during the Renaissance.
The construction of Old St. Peter's followed the Roman basilica plan, incorporating a transept to create a cruciform shape, which was revolutionary in the design of Christian churches at the time. This architectural heritage influenced the grandeur of the new basilica that was to overshadow its predecessor. The ambition to build new churches, monuments, and avenues, as well as the reconstruction of Rome during the beautification projects of the 17th century, reflect the Catholic Church's response to the challenges it faced during the Reformation and its desire to showcase its influence and continued significance.