The Church has defined that heretics are the gates of Hell which Our Lord mentioned in Matthew 16!
Pope Vigilius, Second Council of Constantinople, 553: “… we bear in mind what was promised about the holy Church and Him who said the gates of Hell will not prevail against it (by these we understand the death-dealing tongues of heretics)…”
Pope St. Leo IX, Sept. 2, 1053: “The holy Church built upon a rock, that is Christ, and upon Peter… because by the gates of Hell, that is, by the disputations of heretics which lead the vain to destruction, it would never be overcome.”
St. Thomas Aquinas (+1262): “Wisdom may fill the hearts of the faithful, and put to silence the dread folly of heretics, fittingly referred to as the gates of Hell.” (Intro. To Catena Aurea.)
Notice that heretics are the gates of Hell. Heretics are not members of the Church.
 Decrees of the Ecumenical Councils, Vol. 1, p. 113.
 Denzinger, The Sources of Catholic Dogma, B. Herder Book. Co., Thirtieth Edition, 1957, no. 351.
 The Sunday Sermons of the Great Fathers, Regnery, Co: Chicago, IL, 1963, Vol. 1, pp. xxiv.