Liberalizing = Weakening?
Of course some of that diminishing was due to the increasing religious pluralism of the later 17th and early 18th century (now you had your Catholics, you had your Baptists, you had your Anglicans, you had your Scottish Presbyterians, etc.), but it's quite clear that once the church stopped requiring a true conversion experience as a condition of full membership in the church, things got notably whimpier in the Standing Order.
This is a gross simplification and I'm sure that the Boy In the Bands and Fausto and Adam and Chris and LT and others will correct me -- and they should -- but it all makes me think that the liberal church makes a huge mistake in requiring pretty much nothing of its new members. Sign your name, get a nametag, come to a newcomer's dinner, attend an orientation or two... get applauded in church.
What if the minister or some church elders asked, "Why do you want to be a member of this church? Why do you want to join with this faith tradition? What promises will you make to us, and are you ready to hear our promises to you? Are you ready to enter into this covenant with full commitment, joy and preparedness to be changed by the religious life we share here?"
You know how we always scream, "You can't have creedal requirements for membership! Ayiiiiiiiiiiiiieeeeeeeeeee!"?? Well, that's not a creed. It's not even signing onto the Seven Principles. It's a series of questions designed to weed out the folks who aren't ready and need to be further nurtured or mentored in some way, who don't know what they're doing and need to take a ride on the clue bus, or have no intention of really sticking around.
Someone I really like and respect said to me today (and I'm paraphrasing from memory), "I don't think Unitarian Universalism will disappear entirely. There is always going to be the type of person with an interest in generic spirituality who wants to have the intensity of their ethical commitments validated in community."
Not exactly a ringing endorsement for the Living Tradition of our future, but an honest one.