Tent Meetings: A Ministry On The Move
In 1739 with the closing to two churches, George Whitefield at Kingston, Bristol, England, decided to have an "open air" meeting in the fields of Manham Mount to 250 coal miners. The first open air "evangelistic crusade" was birthed. John and Charles Wesley, of Methodist fame, were at first reluctant to the idea, but eventually fully embraced it and became one of the greatest open air evangelist of his time. By the early 1800's Camp Meeting were everywhere. By the 1900's Tent Meetings sprung up everywhere across America. 
I have a friend who as a child in the 1950's attended Oral Robert's Healing Services in his Tent Meeting. Billy Graham "was discovered" conducting a Tent Meeting on the west coast in the 60's. I attended Jesus Festivals in Pennsylvania in the '70's where there were three tents: one for salvation, another for those seeking "the baptism in the Holy Spirit", and a third for healing and deliverance while all other activities were held in the open air stage on the grounds of a farmer's field and later at Agape Camp Farm in central, PA. My local church, CityView Community Church, perched on top of a hill over looking York, PA in the valley below, rented a tent, which when lit up inside could be seen for miles drawing curiosity seekers to its meetings. In the past decade the "Newsboys" an Aussie Christian Band, did a tour in an inflated structure; I guess the modern version of a tent.
Tents are biblical. God never asked for an immobile structure to have His Presence reside; he chose a tent, the Tent of Meeting, as it was called. David wanted a permanent structure that glorified more grandeur than the palace in which he lived. Tents are mobile, and so is the Spirit of God. Temples and church structures are fixed, stationary, unmovable. I guess we can learn from the Circuit Riders and Tents that the Spirit of Evangelism is a mobile spirit, and both existed outside physical "church building structures".