Former President Donald J. Trump often characterizes his presidential campaign as a battle for America’s future. But speaking at a Christian broadcast media convention in Nashville on Thursday, he wrapped that depiction within a stark good-versus-evil framework, portraying his political opponents as part of a “wicked” system.

Mr. Trump also revived his claim that America’s “greatest threat is not from the outside of our country” but “from within,” language that drew alarm last year from experts who saw in it echoes of autocratic leaders.

During Thursday’s speech at the National Religious Broadcasters convention in Nashville, Mr. Trump portrayed the threat as liberals — more specifically, a “radical left, corrupt political class” — whom he broadly cast as intrinsically bent on attacking Christianity.

“Christians, they can’t afford to sit on the sidelines in this fight,” Mr. Trump said. He later added, without offering evidence, that liberals were persecuting Christians because “they know that our allegiance is not to them. Our allegiance is to our country, and our allegiance is to our creator.” (There are many Christians who are Democrats.)

Before running for office, Mr. Trump made little show of being particularly religious, which he acknowledged early in his speech, and he does not often give faith-focused speeches on the trail. But evangelical voters in 2016 were drawn to his populist message and his pledges to appoint “pro-life” judges, and they have remained loyal to him.

During his third run for office, Mr. Trump has often cast himself as a staunch defender of the Christian right. He also often boasts of his appointment of three justices to the Supreme Court who, in 2022, voted to overturn Roe v. Wade.