In each case, the fires were set at an exterior door and when no one was at the facility, according to a joint statement from the St. Louis Police and Fire Departments and the Bureau of Alcohol, Tobacco and Firearms.
"We believe that this fire-setting activity is meant to send a message," the statement said. It didn't specify what the message was.
The six fires were set over 10 days. All were at predominantly black churches, CNN affiliate KMOV reported.
"I was very thankful in that the potential for more damage was there," the Rev. Roderick Burton with the New Northside Missionary Baptist Church told CNN's Brooke Baldwin.
The fire department put out a fire at the front door of his church.
"Our biggest concern is for whomever is doing this. We want them to get help. We want them to stop absolutely, and I think people would like to know what is the motivation behind it," he said.
The Rev. Earl Nance Jr. described the fires as a travesty.
Although his Greater Mount Carmel Baptist Church wasn't among the churches targeted, he said, "When this kind of thing happens to one or more of our churches we must all come together."
Authorities are investigating, but haven't determined if the fires are connected.
"If you have observed anyone who has recently expressed anger or frustration with our religious community or with these particular churches, we ask that you contact us," the statement said. "If you have noticed anyone or anything suspicious or unusual in the areas near the churches please contact us."
A $2,000 reward is being offered.