Let us begin with the presumption of law as it applies to converts. The practice of receiving a validly baptized person into the Church requires absolution from the censure of excommunication for heresy be made after the convert publicly professes his faith.
In the United States Pastors have been given special faculties to receive validly baptized converts into the Church and provided the procedure as outlined briefly above. Canon 2314 provides that the excommunication for heresy is reserved in a special manner to the Apostolic See. However, the same Canon provides that the Bishop of the Diocese may absolve his subjects, which is an exception to the general law.
Now one would presume that those who were never Catholic would only innocently and ignorantly adhere to heresy, because the had not been previously exposed to the truths of the Catholic Faith, which they readily accepted when they began the conversion process. And yet, the Church requires them to be absolved from the excommunication for heresy.
Saint Augustine says: “Baptism is the privilege of the true Church, and so the benefits, which flow from Baptism, are necessarily fruits, which belong only to the true Church. Children baptized in other communions cease to be members of the Church only when, after reaching the age of reason, they make formal profession of heresy, as, for example, by receiving communion in a non-catholic church.”
Let us consider some specific cases and their resolution:
The Juristvolume 132, page 405: “A young man in my parish joined the Methodist Church at the age of fifteen. He was baptized in it in infancy. At sixteen, through association with Catholic young men in high school, he became a convert to the Church. Does he labor under any irregularity from which a dispensation should be obtained?” (Note heretics become irregular, that is not fit to exercise Holy Orders they have received or to receive Holy Orders.) The answer is: “The young man is also subject to the impediment arising ex delicto from this heresy in accordance with canon 985, 1°. In the internal forum, good faith would excuse him; in the external forum, however, a dispensation should be sought from this irregularity also from the Sacred Congregation of the Sacraments.” In other words, the law presumes guilt, as we shall see.
Homiletic and Pastoral Review in an answer. (Volume 34, Number 7, page 743-4, April 1934) “The question was asked about receiving a baptized non-Catholic in the Church. The usual procedure was followed with absolution from the excommunication for heresy. However, in the accompanying confession the woman confesses to having had an abortion, which is also an excommunicable offense. Let us read part of the answer: Practically there is no difficulty about censure for the sin of abortion or any other offense which the Code of Canon Law punishes with a censure, because ignorance excuses from censures (See Canon 2229, paragraph 1), and it is reasonable to assume that ordinarily the convert was ignorant of the regulations of the Church. One may object and ask why then must we insist on absolving from censure because of heresy when the convert knows nothing more about that censure than he knows of other censures. There is a difference between a public profession of faith contrary to the teaching of the Church and sins committed in one’s private life. The one is a public affair; the other is a matter of conscience only. In public violations of the rules of the Church the public authority cannot but judge that the violation was done with full knowledge, and the burden of proof that it was done in good faith rests with the one who appears to be guilty. In many instance he may not be able to prove good faith, and he will be considered guilty.”
Note that in public violations of the rules of the Church, the Church is obliged to presume formality, that is the the violation was done willfully and with due knowledge.