There is a number of straw man arguments the author makes.
“Over the last several years I have encountered a fair number of Christians who claim they are “spiritual but not religious.” In other words, they do not identify with a particular Christian denomination, using the Bible alone to guide their faith. It’s an ideology that says religious institutions are outdated and unnecessary.”
and
“The issue is whether or not one can do this privately, reading only Scripture and coming to their own conclusions on theological matters, or whether one must submit to some authority outside of themselves.”
The pretense here is people are using ONLY “their own” authority and are not listening to any authority outside their own. In fact, in the area of human guidance non-demominational Christians can be following a number of human “authorities” of one denomination or more as well as theologians and pastors not particularly associated with a single denomination in their writing, AND with the one authority in scripture G-d. All the writer is really complaining about is the non-denominational are not listening to some “authority” he approves of.
“In Matthew 16:18, Jesus says to the apostle Peter, “you are Peter, and on this rock I will build my church.” Catholics believe that in this verse Jesus is bestowing on Peter a position of authority from which the office of the pope is derived. But even if the “spiritual” Christian has problems with this belief, there is no escaping the fact that Christ intended his Church to be both visible and authoritative.”
And for Christians who do not accept the Roman misinterpretation - formulated to serve the human institution given the emperor’s blessing, and for all non-demominational Christians “authoritative” does not have to conform to a certain denomination and “visible” is the spiritual Church made visible and known by the Christian life of Christian believers, in and out of any denomination.
Though the article may attempt to use "spiritual but not religious" in order to escape the charge of elitism, it is obvious what his premise is..
In fact, in the area of human guidance non-demominational Christians can be following a number of human “authorities”
To which the RC will reply than an ultimate supreme infallible authority is necessary, but which is nowhere promised shown as necessary (despite RC extrapolative attempts), but instead God often provided and preserved Truth by raising up men from without the magisterium to correct it. And thus the church began and faith has been preserved.
Moreover, RCs no more have an infallible interpretation for their supreme infallible authority than evangelicals do for theirs, and Catholicism exists in schism and sects, abounding in variant interpretations
In addition, Caths whom Rome treats as members in life and in death overall are less unified than evangelicals , and as what an entity really believes is shown by what it does and effects, (Mt. 7:20; Ja. 2:18) so this constitutes much what Rome really believes.