That idea percolated until 2012. At an event at Harvard, Mr. Jarry, who was taking graduate classes there, met the man who became “Mr. Greaves,” a man who, when not participating in Satanic Temple activism, is often called Douglas Mesner. He is now 39 years old and says he “does some odd jobs” for a living. Mr. Jarry and Mr. Mesner bonded over a shared distaste for organized religion and an inclination to fight back with mischief.

At the time, Florida’s governor, Rick Scott, was pushing a bill to allow voluntary prayer at public school functions. After the bill passed, the two traveled to Florida to make their feelings known.

“So we created this mock rally in support of Rick Scott,” Mr. Jarry recalled, “where we were coming out to say how happy we were because now our Satanic children could pray to Satan in school.”

Mr. Mesner stood behind Mr. Scott on the steps of the state Capitol, holding a banner proclaiming, “Hail Satan! Hail Rick Scott!”

At the time, the name associated with the Satanic Temple’s email account was “Lucien Greaves.” When it became clear that the news media would want to interview someone from the organization, Mr. Mesner adopted Lucien Greaves as his pseudonym. In 2013, Vice magazine outed Mr. Greaves as Mr. Mesner, and since that time he has answered to both names — although in fact, “Douglas Mesner” is also a pseudonym, one he has used for many years. He, too, asked that his legal name not be published, to prevent threats to his family.

Mr. Mesner insists that his attraction to the Satanic label is not just opportunistic. Although he, like Mr. Jarry, is an “atheistic Satanist,” meaning that he no more believes in a literal Satan than he does in a literal God, he finds special meaning in Satanism, which represents to him the solidarity of outsiders, those judged and excluded by the mainstream.