Any baptized person who publicly and pertinaciously rejects a dogma of the Catholic Church— therefore becoming separated from God and His Church, is a formal heretic, and anyone who openly supports such a person in his heresy sins gravely.
“A man that is a heretic, after the first and second admonition, avoid: Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment.”—Epistle to Titus 3:10-11,Douay-Rheims Bible
Based upon this admonition, Catholics have long been instructed to shun the preaching, writings, and assemblies of heretics, yet as St. Paul the Apostle here does not specify to what extent each sort of heretic is to be avoided, the Catholic Church has provided guidance through her consistent disciplinary decisions since antiquity.
“With heretics one must neither pray nor sing psalms. Whoever shall communicate with those who are cut off from the communion of the Church, whether clergy or laic: let such be excommunicated.”—Canons 72-73,Regional Council of Carthage IV, 398
Such heretics congregate in obviously non-Catholic "churches" or conventicles, properly called conciliabules. Throughout history, the Church has issued prohibitions against supporting groups (such as the Lutherans and Anglicans) that have been notoriously heretical either in law (explicitly being declared excommunicated) or notorious in fact (openly rejecting the Catholic faith without any concealment). The ecumenical council at the Lateran clarifies that these rules apply specifically once the Church has denounced these heretics as such.
“Moreover, we determine to subject to excommunication believers who receive, defend or support heretics… If any refuse to avoid such persons after they have been pointed out by the Church, let them be punished with the sentence of excommunication until they make suitable satisfaction.”—Constitution 3, “On Heretics,"Pope Innocent III, Fourth Lateran Council, 1215
The Church teaches that a delict, such as heresy, is considered notorious in fact only when it cannot be concealed by even dishonest arguments; hence, the obligation to avoid a heretic only applies if he has been officially declared so, or if he is so notorious that he cannot somehow conceal or excuse his crime in law. The latest canon law of the Roman Church contains the distinctions between public, notorious in law, and notorious in fact: a delict is notorious in fact when it is public and so notorious that it can in no way be concealed or excused.
“A Crime is public: (1) if it is already commonly known or the circumstances are such as to lead to the conclusion that it can and will easily become so; (2) Notorious by notoriety of law, [if it is] after a sentence by a competent judge that renders the matter an abjudicated thing, or after a confession by the offender made in court in accord with Canon 1750; (3) Notorious by notoriety of fact, if it is publicly known and was committed under such circumstances that no clever evasion is possible and no legal excuse could excuse [the act] (4) Occult, if it is not public; materially occult, if the delict is hidden; formally occult, if imputability [is not known]…” —Canon 2197 § 1-4,Code of Canon Law, 1917
Furthermore, the indefectible Catholic Church has occasionally allowed Catholics to exchange the sacrament of matrimony with heretics, indicating that communication in a sacrament with a heretic does not necessarily entail communication in the sin of a heretic, as the transgression rests in disobeying the very interdiction of the Church. For the sacrament of the Eucharist, similarly, theologians such as Navarro, Sanchez, Suarez, and Hurtado postulate that it can indeed be permissible to receive communion from heretical, excommunicate, or sinful priests, and to hear Mass said by them.
“[...H]eretics, schismatics, and excommunicates, have been forbidden, by the Church's sentence, to perform the Eucharistic rite. And therefore whoever hears their Mass or receives the sacraments from them, commits sin. But not all who are sinners are debarred by the Church's sentence from using this power: and so, although suspended by the Divine sentence, yet they are not suspended in regard to others by any ecclesiastical sentence: consequently, until the Church's sentence is pronounced, it is lawful to receive Communion at their hands, and to hear their Mass.”—Question LXXXII, Article IX, Part III,St. Thomas Aquinas, Summa Theologiae
The Church knows that the sin of abusing her sacraments is committed by the undeclared heretics themselves, not by the Catholics who recur to the sacraments, so she explicitly reiterates that the faithful are free without scruples to receive the sacraments from the hands of those undeclared heretics who abuse them.
“To avoid scandals and many dangers and relieve timorous consciences by the tenor of these presents we mercifully grant to all Christ's faithful that henceforth no one henceforth shall be bound to abstain from communion with anyone in the administration or reception of the sacraments or in any other religious or non-religious acts whatsoever, nor to avoid anyone nor to observe any ecclesiastical interdict, on pretext of any ecclesiastical sentence or censure globally promulgated whether by the law or by an individual; unless the sentence or censure in question has been specifically and expressly published or denounced by the judge on or against a definite person, college, university, church, community or place. Notwithstanding any apostolic or other constitutions to the contrary, save the case of someone of whom it shall be known so notoriously that he has incurred the sentence passed by the canon for laying sacrilegious hands upon a cleric that the fact cannot be concealed by any tergiversation nor excused by any legal defense. For we will abstinence from communion with such a one, in accordance with the canonical sanctions, even though he be not denounced.”—“Ad Evitanda Scandala,”Pope Martin V, Council of Constance, “ 1418
Cardinal de Lugo cites the papal bull as he affirms the permissibility of receiving sacraments from priests who hold heretical positions while noting that the virtue of charity compels one, when possible, to impede the sin of the heretical minister administering unworthily.
“So as these heretics are not declared excommunicates or notoriously guilty of striking a cleric, there is no reason why we should be prevented from receiving the sacraments from them because of their excommunication...”—Disputatio XXII,Cardinal John de Lugo S.J., Tractatus de Virtute Fidei Divinae, 1646
Priests who administer valid traditional sacraments, even if they retain communion with heretics and propagate what are manifestly grave theological errors, do not need to be avoided until they are declared as such by the Church. This has been the practice of the Church throughout history. The absolute obligation to definitively avoid someone such as the then-only suspected heretic Nestorius comes with a declaration, such as the one which St. Cyril of Alexandria asked of Pope Celestine.
“We have not confidently abstained from communion with him (Nestorius) before informing you of this; condescend, therefore, to unfold your judgment, that we may clearly know whether we ought to communicate with him who cherishes such erroneous doctrine.”—“The Council of Ephesus: Historical Introduction”The Nicene and Post Nicene Fathers, 1996.Under the Anglican persecutions, there were priests who administered true Communion to remnant Catholics in the traditional rite of the Church before going on to participate in the sacrilegious Anglican rite. Edward Rishton, a Catholic priest who was tried and condemned to death with Bl. Edmund Campion, recounts how faithful Catholics received Communion from such heretics in their state of necessity without succumbing to their heresy or receiving the invalid bread which the heretical priests gave to others.
“At the same time they had Mass said secretly in their own houses by those very priests who in church publicly celebrated the spurious liturgy, and sometimes by others who had not defiled themselves with heresy; yea, and very often in those disastrous times were on one and the same day partakers of the table of our Lord and of the table of devils, that is, of the blessed Eucharist and the Calvinistic supper.”—Fr. Edward Rishton, The Rise and Growth of the Anglican Schism, 1585
Fr. Rishton never condemned these people as heretical, and neither does the Church, whose discipline explicitly allows Catholics to do likewise.
“[…T]he faithful may for any just cause ask the sacraments or sacramentals of one who is excommunicated, especially if there is no one else to give them… But from an excommunicated vitandus or one against whom there is a declaratory or condemnatory sentence, the faithful may only in danger of death ask for sacramental absolution according to canons 882, 2252, and also for other sacraments and sacramentals in case there is no one else to administer them.”—Canon 2261.2-3,Code of Canon Law, 1917
There can be no doubt that receiving grace-giving sacraments from undeclared heretics in itself is no sin at all. Faithful Catholics should rather die than commune with heretics officially condemned by the Church such as the so-called Anglicans, Patriotic Catholics, or Orthodox, yet they do not need to deprive themselves of the graces of the sacraments just because the only available minister of those sacraments might be a heretic waiting to be declared as such by the Church in the future. The undue burden that schismatics like Fr. Anthony Cekada and Richard Ibranyi purport to impose on the faithful, banning them from receiving sacraments from undeclared heretics, can be dismissed as contrary to the discipline of the universal Church.