Charlie Kirk was 18 in the summer of 2012 when he walked into a Starbucks in the suburbs of Chicago and plopped his backpack on the floor.

He had recently graduated from high school and was sorting out his future after being rejected from the U.S. Military Academy at West Point. He had come to meet a local Illinois group, Patriots United, focused on issues like lower taxes and choice in school education during the Tea Party era.

Maida Korte, 71, a leader of the group, recalled that the conversation turned to their shared foundational Christian values, as it often did with him.

“He never thought of it as, ‘I’m going to go into the spiritual arena and talk about political things,’” she said. “He was, ‘I want to talk about spiritual things, and in order to do that, I have to enter the political arena.’”

That fusion of Christianity and politics reached a level on Sunday in Arizona that Mr. Kirk could only have imagined that summer day years ago. He did not live to see it, after his recent assassination at age 31. But in a stadium full of mourners, including President Trump, Vice President JD Vance, House Speaker Mike Johnson and tens of thousands of others, the success of his ideals of a politically active, conservative Christianity was omnipresent.