Gaustad died March 25 of natural causes at an assisted-living facility in Santa Fe, N.M., said a daughter, Susan.
"He did incredible work as kind of a geographer of American religion," said Leigh Eric Schmidt, a Harvard University professor of the history of religion in America. "He did this big atlas about what the American religious scene looked like, county by county, over a long stretch of history."
The result was a first, the "Historical Atlas of Religion in America," which provided Gaustad with a rare scholarly surprise when he completed it in 1962.
"I thought we would be a much more homogeneous country religiously," he told the Santa Fe New Mexican in 1996. "We aren't."
Gaustad — which rhymes with "ousted" — was also prominent among a handful of historians in the mid-20th century who broadened the study of religion in the United States from theological schools to a wider university setting, Schmidt said.
After he joined UC Riverside in 1965, Gaustad was instrumental in establishing the university's religious studies program, said June O'Connor, a UC Riverside professor of religious studies who worked with him.
"He was a scholar's scholar," O'Connor said. "Scholars really appreciated his work and the scope and expanse of his work and productivity."
Yet Gaustad wanted his work to speak to broader public culture, "to make the historian's voice count in public debates of church and state," Schmidt said.
Gaustad was a leading expert on America's Colonial period, especially in the areas of religious liberty, pluralism and dissent.
He wrote more than a dozen books on religious history, many of which were regarded as seminal works. They included "A Religious History of America," a popular 1966 text that was last updated, with Schmidt's help, in 2002; "Church and State in America," published in 1998; and "Roger Williams," a 2006 biography of the theologian who helped found Rhode Island and the Baptist church.
A year after Gaustad published "Sworn on the Altar of God" (2001), a religious biography of Thomas Jefferson, he testified as an expert witness in the legal case brought by the American Civil Liberties Union and other groups against an Alabama judge who refused to remove a monument of the Ten Commandments from the state courthouse.
When an attorney asked who Jefferson was referring to when he wrote that liberties are a gift from God, Gaustad said, "The God of nature," which is "the God we see around us," the Associated Press reported in 2002.
"Not the God of the Bible?" he was asked.
"Not for Jefferson," Gaustad replied.
Edwin Scott Gaustad was born Nov. 14, 1923, in Rowley, Iowa, the youngest of three sons of Sverre and Norma Gaustad. A lifelong Baptist, he grew up in Houston.
During World War II, he served in the Army Air Forces as a bombardier stationed in Italy.
He earned a bachelor's degree in history and English in 1947 from Baylor University. At Brown University, he studied under noted early American history professor Edmund S. Morgan and received his master's in 1948 and his doctorate in 1951 in the history of religion.
After teaching at Georgia's Shorter College, Gaustad was a humanities professor at the University of Redlands from 1957 to 1965 before joining UC Riverside.
A longtime resident of Redlands, he moved to Santa Fe to be near his grandchildren after retiring from UC Riverside in 1989.
He was "affable, gentle and modest," Schmidt said, and remained emphatic about the need for the separation of church and state.
"The most horrifying but most obvious" reason for it was to prevent "religious warfare — because that was true of England in the 17th century," Gaustad told the Charlotte, N.C., Observer in 1999. "Any time you put the power of the state … behind conformity and religion, you're calling for religious persecution."
"From our relaxed point of view," he said, "We say, 'Oh, that could never happen.' But the fact is it happened over and over again."
Helen, his wife of 63 years, died in 2009.
In addition to his daughter Susan of New York City, he is survived by a son, Scott, of Ukiah, Calif.; another daughter, Peggy of Santa Fe; four grandchildren; and one great-grandson.