Mr. Boyd, 49, who preaches in blue jeans and rumpled plaid shirts, leads a church that occupies a squat block-long building that was once a home improvement chain store.

The church grew from 40 members in 12 years, based in no small part on Mr. Boyd’s draw as an electrifying preacher who stuck closely to Scripture. He has degrees from Yale Divinity School and Princeton Theological Seminary, and he taught theology at Bethel University in St. Paul, where he created a controversy a few years ago by questioning whether God fully knew the future. Some pastors in his own denomination, the Baptist General Conference, mounted an effort to evict Mr. Boyd from the denomination and his teaching post, but he won that battle.

He is known among evangelicals for a bestselling book, “Letters From a Skeptic,” based on correspondence with his father, a leftist union organizer and a lifelong agnostic — an exchange that eventually persuaded his father to embrace Christianity.

Mr. Boyd said he never intended his sermons to be taken as merely a critique of the Republican Party or the religious right. He refuses to share his party affiliation, or whether he has one, for that reason. He said there were Christians on both the left and the right who had turned politics and patriotism into “idolatry.”

He said he first became alarmed while visiting another megachurch’s worship service on a Fourth of July years ago. The service finished with the chorus singing “God Bless America” and a video of fighter jets flying over a hill silhouetted with crosses.

“I thought to myself, ‘What just happened? Fighter jets mixed up with the cross?’ ” he said in an interview.

Patriotic displays are still a mainstay in some evangelical churches. Across town from Mr. Boyd’s church, the sanctuary of North Heights Lutheran Church was draped in bunting on the Sunday before the Fourth of July this year for a “freedom celebration.” Military veterans and flag twirlers paraded into the sanctuary, an enormous American flag rose slowly behind the stage, and a Marine major who had served in Afghanistan preached that the military was spending “your hard-earned money” on good causes.