I would say that what these poll takers conclude to be Christianity is a very fuzzy area. The "face" which many see and describe as a group [named Christianity] striving to follow Christ, has probably always been a false "face" to God. About 2,000 years ago, Apostle Paul wrote this verse:Dig deeper you will find many netlist are cults and apostate. Also look at the list of those in decline.
Just calling oneself Christian does not make it so. You check out all the facts Christianity is declining and has been years.
"For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known." I Cor 13:12
Most believers in any measure when he wrote it were seeing "through a glass, darkly". What reason do we have to believe that most people have not remained in a very similar state? Man's history? Remember who wrote man's history: Was it likely not some of those who conduct polls today on which too many people draw conclusions and take action instead of simply being led by the Holy Spirit?