Frank Newport at Gallup:
The overall religious composition of the adult population in the U.S. is about the same as it was last year, with the significant majority of Americans identifying with either the Protestant or Catholic faith, along with about one in five who say they have no formal religious identity. Americans are also about as religious this year as last year, based on their self-reported church attendance and the importance of religion in their lives. Two-thirds of Americans are either highly or moderately religious.
The basic pattern of association between Republican partisan identity and higher levels of religiosity continued in 2017, with the associated finding that Trump does better among those who are highly religious than among those who are less religious. The relationship between religiosity and Trump approval is primarily found among non-Hispanic white Americans; nonwhites give Trump very low job approval ratings regardless of their religiosity.