How did the church of Jesus grow for the first 1900+ years of its existence without any megachurches around?
That’s how long it took for the first megachurches to appear on the landscape. They’re the new kid on the church block. Until the middle of the 20th century, the relentless growth of the church moved forward, not through growing bigger churches, but almost exclusively through the multiplication of smaller congregations. And that’s where most of the growth of the church still happens today.
The church must always grow. The church will always grow. We know that because Jesus said he’d do it. In some places, that means larger congregations. In most places that means more healthy small congregations.