There is a difference between holding loosely to beliefs where Scripture is not clear and holding to demonstrably false beliefs where the Bible is abundantly clear. When one denies the clear biblical teaching of Scripture, it becomes an issue of rebellion against God and His word placing one in serious danger of God's judgment.
There are quite a few very disturbing findings in that survey.
As for this:
"Another false belief that the majority of professing Christians believe is that church membership doesn't matter."
What defines "church membership", and what does it mean to "join a local church"? I looked at the survey but they do not define those terms.
I agree. Doctrine defines a church. Sola Scriptura.
judgement begins at the house of God