Let’s not overlook that not long after Adams a significant number of Congregationalists decamped for the Unitarians, although granted 19th Century Unitarians were much more “Christian” than the modern variety.
Universalism has always been deeply rooted in Boston, and it’s always been a stumbling block to sharing the Gospel there. The intent of the piece is that people understand WHY Boston must be evangelized from a Christian perspective, versus from Boston’s perspective (which says “we’re fine without Christ, give us Zoroastrianism or any other pagan belief but not Christ).