At the Public Religion Research Institute, they have some figures out on Religious Change in America’s Swing States, including Iowa:
Iowa (+8 Religiously unaffiliated)
Iowa has experienced a dramatic growth of religiously unaffiliated residents in the last seven years. Nearly one in four (23 percent) Iowans are religiously unaffiliated today, compared to 15 percent in 2007. The state has also experienced a proportional decline in its white Christian population, most notably among white Catholics. The white Catholic population in the state dropped from 21 percent in 2007 to 16 percent in 2014.
See the full post here: Religious Change in America’s Swing States.