The same proportion, 73 per cent, also agree that religious faith is very important in their life, with 52 per cent strongly agreeing and 21 per cent somewhat agreeing.
However, only 31 per cent said they attended church at least once a month, a measure used by Barna to classify people as "practising Christians".
Amid an ongoing debate in the US about whether it is a "Christian country" – a debate which mirrors a similar one in the UK – around 48 per cent of those surveyed fell under the category of "post-Christians".
These are people who do not take part in activities such as Bible reading, prayer, and church attendance, and have disbelief in God or identify as atheists or agnostics. Individuals were catagorised as "post-Christian" when at least 60 per cent of their responses were positive on the scale testing lack of involvement in church practices and feelings of disbelief.
The study also found that Christians are more generous than secular Americans, with as many as 96 per cent of practising Christians donating to churches and non-profit organisations, while only 60 per cent of atheists and agnostics did so. Some 94 per cent of practising Christians donated to churches.
Three quarters of Americans said they prayed to God over the last week, and about 35 per cent had attended church in the seven days before they were polled.
About 35 per cent of the Americans were catagorised by Barna as born-again Christians, 23 per cent were Bible-minded, and some 7 per cent professed evangelical faith.
Evangelicals, described by Barna as a "small but influential" category, must meet the 'born again' criteria plus seven other conditions. Barna said: "These conditions include saying their faith is very important in their life today; believing they have a personal responsibility to share their religious beliefs about Christ with non-Christians; believing that Satan exists; believing that Jesus Christ lived a sinless life on earth; asserting that the Bible is accurate in all that it teaches; believing that eternal salvation is possible only through grace, not works; and describing God as the all-knowing, all-powerful, perfect deity who created the universe and still rules it today".
Barna catagorised individuals as either "churched" – active churchgoers who have attended a church service within the past six months, not including a special event such as a wedding or a funeral – and "unchurched", who have not attended such a service. Under these definitions, a slight majority of adults – 55 per cent – are churched, while 45 per cent are unchurched.
The study, aimed at testing what Barna called the "state of the church" in America, was conducted during the first half of 2016 with over 5,000 adults interviewed via internet and telephone across the US.
The Barna group was established 30 years ago and says it has conducted "more than one million interviews over the course of hundreds of studies" on the issue of faith and other issues in the US