Congregationismn.
See quot. 1884.Apparently an isolated use.
ΚΠ
1884 Nonconformist & Independent 22 May (Suppl.) 1/1 If the church was swamped by subscribers, that was Congregationism, and not Congregationalism.
This entry has not yet been fully updated (first published 1891; most recently modified version published online September 2019).
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n.1884