I don’t believe those who have been truly regenerated, born from above, are scared to confess Christ to pollsters. I agree with Russell, as we lose influence on the culture, it’s the cultural Christians we are losing.
“But whosoever shall deny me before men, him will I also deny before my Father which is in heaven.”
It could also be a matter of population change. Old Christians are dying. Children are being born into single family households that have rejected God.