But the failure of the voluntary principle in regard to maintaining Protestant Institutionalism is not because of some abuse or misapplication of the principle. It's because the principle itself is inherently flawed and set up to fail (that is, if one desires to maintain Protestant Institutionalism anyway). It's not designed - and its goal is not - to favor one religion over another, or to favor religion at all. The evolution from Protestant Institutionalism to secular humanistic/Marxist/atheistic institutionalism is not a failure of the voluntary principle. It's actually the voluntary principle working exactly as advertised. As the nation's religion (or its rejection of religion altogether) changes over time, its institutions change to reflect that. Authority/sovereignty is in "we the people", not the Word of God. You may not like it that Marxists/atheists have taken over our institutions, but why would you expect anything different? Without any mechanism in place - such as the official recognition of Christianity as the official religion of the State - to restrain it, what leg do you have to stand on in trying to maintain the dominance of Christianity in our institutions? One could even argue that the voluntary principle - "neutrality" and the toleration of all religions as enshrined in the 1st amendment - has been a great success based on the fact that it has removed the dominant influence of one religion (Christianity) from our institutions, culture, ethics, politics, etc.
It's different with the EP. Listen to Rev. Beer's explanation of what happened in Scotland and other areas (it's a mere 6 minutes of your time). It was because they rejected the EP, not because of a failure of the EP. But we have abortion on demand, same sex marriage and drag queen story hour in the US precisely because the majority of the nation has embraced the voluntary principle, not because it has been rejected or misapplied.
Start at about the 1:00:15 mark and go to about 1:06:45: