Engels’s quote in context:

A revolution is certainly the most authoritarian thing there is; it is the act whereby one part of the population imposes its will upon the other part by means of rifles, bayonets and cannon — authoritarian means, if such there be at all; and if the victorious party does not want to have fought in vain, it must maintain this rule by means of the terror which its arms inspire in the reactionaries.

Engels was talking about revolutionaries’ suppressing reactionaries in order to defend revolutionary gains.

Source: “On Authority”, in Marx/Engels Collected Works, volume 23, 1988.

EDIT: As if to add insult to injustice, Jessie had her “friend Steve Shives who is a self-professed reformist liberal” read that Engels’s misquotation.