Context: The Bishop of Constantinople was allowing heretics to meet and present their views. The heretics were led by Apollinaris, who denied the Trinitarian faith (though in words he held some of it), denied the full humanity (though in words he deceived Pope Damasus and others for a while).
Gregory writes to Nectarius, Bishop of Constantinople. Christology of the Later Fathers, p. 231
“Now, if they who hold such views have authority to meet, your Wisdom approved in Christ must see that, inasmuch as we do not approve their views, any permission of assembly granted to them is nothing less than a declaration that their view is thought more true than ours. For if they are permitted to teach their view as godly men, and with all confidence to preach their doctrine, it is manifest that the doctrine of the Church has been condemned, as though the truth were on their side. For nature does not admit of two contrary doctrines on the same subject being both true. How then could your noble and lofty mind submit to suspend your usual courage in regard to the correction of so great an evil? But even though there is no precedent for such a course, let your inimitable perfection in virtue stand up at a crisis like the present, and teach our most pious Emperor, that no gain will come from his zeal for the Church on other points if he allows such an evil to gain strength from freedom of speech for the subversion of sound faith.”