Mary of Holyrood may smile indeed,
Knowing what grim historic shade it shocks
To see wit, laughter and the Popish creed,
Cluster and sparkle in the name of Knox.
–G. K. Chesterton
That little poem by a large man, c. 1932, is part of the wider bewilderment with which Chesterton beheld the modern(ist?) Protestant, something he brings out fully in his book The Thing: Why I Am Catholic (my favourite quotations here).
In The Thing, Chesterton is not impressed by the Protestants he sees around him, for they seem unable to properly define a Protestant, for one thing. For another, the virtues they find in the writings of famous Protestants such as Milton and Bunyan are not things that are peculiarly Protestant. Rather, they are things that he sees as being peculiarly Catholic.
I would argue with dear Mr. Chesterton, however, that these things are not simply Catholic but more properly catholic. That is to say, the things that modern Protestants love about Milton and Bunyan, Shakespeare and Donne, may not be peculiarly Protestant things, but they are not Catholic in the sense of Roman/Romish/Papist/what-have-you, but catholic in the sense of universal — they are part of the common store of all Christians everywhere at all times, be they Orthodox, Catholic, or Protestant.
Nevertheless, GKC has a point here. Chesterton is right in berating modern Protestants for not really liking the things that, historically, define them as Protestants. In the forefront of his mind seems to have been a dislike of some form of Calvinistic Predestination, which is a bit amusing, given that Predestination itself is as much part of the Catholic theology as Protestant, both groups being inescapably Augustinian despite their best efforts in recent times.
There is something troubling about a group that dislikes the things that define it and loves only those things that it has common with everyone else. Why, exactly, should one be part of said group? Why be a Protestant if the only things you like about Protestantism are things Protestants hold in common with Roman Catholicism? Why be a Christian if the only things you like about Christianity are things Christians hold in common with all religions?
While I do not argue we must all adhere strictly to the confessional documents of the 16th and 17th centuries, I do think we should take them into account, we should figure out if these adjectives of old — Protestant, Catholic, Lutheran, Anglican, Reformed, Orthodox — actually refer to our particular set of beliefs or if we are merely followers of the heresy of Scholiastism (insert your own name before the -ism).