Like most Americans who live in major East Coast and Midwestern cities, I have always belonged to parishes with century-old church buildings that fit a certain style: tall, long, ornate, likely gothic. When I first began researching the more modern-looking Catholic churches that began cropping up in the postwar boom, I was drawn to the major buildings which, like St. Norbert’s Abbey in De Pere, Wisconsin, (dedicated 1959) were intended by their builders to last for a millennium. As I visited churches around the country, however, I found myself increasingly curious about another kind of building, replete with folding walls, cheap carpet and movable chairs. I also researched worship spaces of the 1960s and 1970s I could read about, but not visit, since the storefronts, apartment buildings and offices they had inhabited were long repurposed. Why would anyone prefer such a space to the grandeur of, say, Marcel Breuer’s St. Francis de Sales parish church in Muskegon, Michigan? Had these communities simply lacked money? Ready cash is always welcome for any parish building project. But as I explored the churches of the Second Vatican Council era, I found that what initially looked makeshift had in fact responded to some of the most important Catholic discussions of the 1960s. By 1969, as Minnesota pastor Father Leo Howley told his congregation in a sermon for their new church’s dedication, Catholics had been “forced … to answer a question we have taken for granted all our lives. … What is a church?” Not every American Catholic relished asking this question. But those who found it more exciting than disturbing often coalesced around the architectural value of flexibility. Groups that chose flexible worship spaces sometimes mentioned financial constraints, though less because of poverty and more as resistance to responsibility for large debt and maintenance expenses when there were more pressing community needs. More often, though, flexibility seemed like an answer not to monetary woes but to an atmosphere of post-Vatican II liturgical uncertainty: When, if ever, would the liturgy stabilize? Two monks from St. John’s Abbey, ground zero for American liturgical reforms, wrote in 1963 that “it is quite clear that Vatican Council II will have a profound effect on the design of the church,” and it would not be “practical to do nothing when the times demand action.” Yet, they continued, “many people seem to be confused as to just what this effect will be.” It took another decade before the Mass had settled into its contemporary form. Those who built churches in that decade had to contend with the possibility of a highly fluid future. They were well aware of communities that had built long, narrow churches with altars against the rear wall of the sanctuary in the 1950s, only to undertake expensive post-conciliar renovations within 10 years. Flexible spaces also sought to encourage lay participation in the Mass. For the pre-Vatican II liturgical movement, that meant minimizing visual distractions and shrinking the distance between the pews and the altar, enabling the congregation to see and hear the priest and to focus on the liturgy rather than private prayer. But by the late 1960s, some concluded that the congregation should also “participate” in making the worship space itself. A few congregations, like the Catholic student group at New Mexico State University in Las Cruces, took pride in designing and building their church spaces themselves. Others, however, encouraged congregations to remake the liturgical space each week by rearranging seating and creating rotating or disposable visuals, both engaging them in offering the Mass and preventing spiritual stagnation. The artist Immaculate Heart of Mary Sister Corita Kent commented that “the people who worship in each church” should “do the decoration,” which should “always be temporary.” Some new churches, like St. Leo’s, Pipestone, Minnesota, came not with permanent art or pews but with movable chairs and hooks, clips and tracks for the easy hanging of artwork. Constantly enlisting parishioners’ creativity to rearrange the space enabled them to actively participate not just during the hour of liturgy, but during its preparation. Finally, the unfinished look of some flexible worship spaces turned out to be a deliberate theological choice. Multipurpose spaces of the 1960s, which can be converted from church to meeting room, auditorium or dining hall, among other uses, bear a superficial resemblance to the temporary gym-churches of 1950s parishes. But some post-Vatican II communities felt that convertible space signaled the kind of church they intended to be: open to a future of “uses which we can’t even possibly foretell,” as Troy, New York architect Peter Levatich wrote. What if Catholics reunited with Protestant and Orthodox Christians, or those from many faiths came to worship together? One Minnesota parishioner wrote of her new church, with its many flexible elements, that it did not represent a finished reality, but was “a symbol of hope … conceived, designed, and built in a time of turmoil and change” and pointing towards a “mature, 21st-century community.” It was a church of the “Risen Christ,” journeying toward “the fullness of redemption.” Why should its building be set, as it were, in stone? Many of these flexible spaces have been as fragile as their communities anticipated at the time. Some were utterly transient, intended to serve as worship space for only an hour or two. Others have solidified — folding walls removed, pews added, permanent decorations and liturgical furniture installed as newer generations of parishioners and pastors expressed a preference for stability. But envisioning the spaces as they were, I now see not what they lacked (stained glass, pews, finished walls) but what their designers filled them with: a profound hope, tempered by uncertainty, in a time of great change.