[Kalam]

...late Middle English: from Latin congregat- ‘collected (into a flock), united’, from the verb congregare, from con- ‘together’ + gregare (from grex, greg- ‘a flock’).

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That makes sense, as a ‘grouping together’ It even denotes the idea of a flock for congregation. Where Christ is the Sheppard of His flock, which has become many congregations.

[ichabod1]

CONGREGAVIT NOS IN UNUM CHRISTI AMOR