“If Evangelicals make up a majority of Iowa caucus voters, then they must have a far greater turnout than any other religious faith in Iowa.”
You are assuming that Iowans have an equal propensity to vote Republican irrespective of their religion, and that turnout at caucuses is akin to that at primaries. Iowa Evangelical Protestants are heavily Republican (particularly the Dutch in the NW corner of the state, which have an outsized influence on GOP contests), and participation in caucuses is so time-consuming that only the most politically active Republicans participate. If white Evangelical Protestants are 25% of Iowa’s population, then it shouldn’t come as a big surprise if they are 51% of GOP caucus goers.
BTW, the most heavily Catholic part of Iowa is the Dubuque area, and while a lot of pro-life Dubuque Catholics participate in the GOP caucuses, the city as a whole (and the entire region, for that matter) votes like 60% Democrat, and I suspect that a majority of Iowa Catholics vote Democrat.
Dubuque County hasn’t voted GOP for President since 1956.