What is the church? Compare the NT church with that of the illegal, imperial, (medieval) ages. What is your definition of Church?
A distinction must be drawn between The Church vs. A Church. The Church consists of Christ's true followers in all the earth and in all time regardless of denomination. A Church is one part of The Church, as a community of Christ Followers fulfilling The Great Commission.
Jesus established The Church as the fulfillment of Judaic prophecy, and bestowed authority on his apostles to establish local churches. A church during the NT period began as a natural extension of post-diaspora synagogues (as house churches) as part of God's divine preparation for world evangelism with a common language, roads, peace, and Gentile spiritual hunger.
Once The Church was identified as separate from Judaism, it was declared illegal by Rome and A Church was then forced to meet secretly to avoid persecution. Curiously, God used the blood of martyrs as the seed of The Church. Christ Followers fled only to plant new churches and people saw the truth of Christ in those dying for it. Authority in The Church passed from the martyred apostles to new bishops, until Christianity was legalized by Emperor Constantine.
Once the official Imperial religion, men slowly made The Church indistinguishable from a complex structured hierarchy of A Church built to model the Empire. Humble churches were replaced by ornate cathedrals seemingly along with the invisible nature and authority of The Church. The Tower of Babel arguably repeated while the empire crumbled, as men fought for church authority and were divided into a confusion of denominations that continues today.
Despite history's lessons, man still debates on what is and how to do Church. We still run the risk of thinking we build The Church and A Church is a just a building, instead of recognizing that God establishes The Church through A Church in unity with other churches carrying out the commands of Jesus regardless of the environmental, political, or demonic forces of the day.