Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon VI

Canon VI.

It is not permitted to heretics to enter the house of God while they continue in heresy.

Notes.

Ancient Epitome of Canon VI.

The holy place is forbidden to heretics.

Aristenus.

Heretics are not to be permitted to enter the house of God, and yet Basil the Great, before this canon was set forth, admitted Valens to the perfecting of the faithful [i.e., to the witnessing the celebration of the Divine Mysteries].

Van Espen.

A heretic who pertinaciously rejects the doctrine of the Church is rightly not allowed to enter the house of God, in which his doctrine is set forth, so long as he continues in his heresy.  For this reason when Timothy, Archbishop of Alexandria, was consulted concerning the admission of heretics to church, answered in the IXth Canon of his Canonical Epistle, that unless they were ready to promise to do penance and to abandon their heresy, they could in no way be admitted to the prayers of the faithful.

Contrast with this Canon lxxxiv., of the so-called IVth Council of Carthage, a.d. 398.