When it comes to speaking of the Incarnation of Jesus, theologians and the authority of the Church are very careful not to speak loosely or confusedly about the very being of the Lord Jesus. He is both the Second Person of the Blessed Trinity and possesses in a union with the essence of a human nature. Catholics acclaim at Benediction of the Blessed Sacrament, “Blessed be Jesus Christ, True God and True man.” The Person of God the Son has attributes as God, and the human nature or the limited being of the Son of God as human also has human characteristics. Interchanging abstract names for one nature to the other nature without qualification leads to serious errors and ambiguity. If someone were to say, “The deity suffered and died,” that would lead to denying the Triune God is immutable and therefore changeable. Likewise, if someone were to assert that Jesus as man is everywhere, is creator, infinite, or omniscient, then he would be asserting a contradiction because a man cannot be everywhere, create, or possess infinity and be omniscient. Otherwise, Jesus would be a hybrid.
Such distinctions are about language and they make certain that the mystery of the Incarnation is not asserted in a bundle of confusions or ambiguities. This is necessary since the Catholic faith is absolutely true and worthy of acceptance. Similarly, but on a lesser level of thought, clarity is essential for understanding moral truth lest the precepts of God are not understood as binding directives to happiness but mere ideals to strive for, more or less. Some commandments are positive and are to be obeyed under the right circumstances of time, place, and ability. For example, if someone is in danger of death by drowning, and I am proximate to that occasion, then I am potentially obliged to save that person. However, if I do not know how to swim, I cannot save a drowning person without drowning myself or attempt to simply by trusting in God. Likewise, I should give honor and physical aid to my parents, but I cannot do so now because I cannot leave my job in a foreign country without jeopardizing the health and welfare of my own family, my obligation to my parents still exists but is put on hold.
However, there are precepts of God, known even by natural reason and a modicum of faith, that always and everywhere under all circumstances are wrong always and everywhere intrinsically evil. Some of these commandments revolve around sexual morality (fornication, adultery etc.), others are interior and exterior evil acts against the theological virtues (doubts of faith, despair against hope etc.). Often, many sins of the sexual sphere are committed out of weakness of fallen nature, or the weakness that has come from previous willful acts of sin, with some knowledge, clear or vague, and occasionally with almost all ignorance.
In a completely corrupt pagan society, it is theoretically possible that persons of some good will with almost complete ignorance may commit fornication, masturbation, homosexual acts or adultery not realizing their evil nature. Their sinning may not be formal but material. If they had a baptism of desire, they might still be in sanctifying grace. St. Thomas Aquinas reminds us that due to unusual circumstances, formal mortal sin may not be committed because of obstacles to freedom. The Catechism says as much:
1735 Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.
Once a penitent or a budding convert is warned of his condition and instructed by true knowledge and makes up his or her mind to cease committing these sexual sins, then a sign of true sorrow is a willingness to abandon sinning and practice works of penance, which are prayer, fasting and works of mercy (spiritual and corporal). Living together without a marital commitment and engaging in sexual acts is necessarily committing the sin of fornication or adultery if one or both are married to someone else. We only know unauthentic acts of sexual intercourse are mortal sins (expelling the life of grace in a soul) from the teaching of the Church because pure reason alone can only say these acts flow from the vice of lust. On the other hand, living in an adulterous situation is not marriage, and engaging in sexual acts for many good reasons with the best of intentions are not authentic marital acts because they lack the bond of a permanent commitment, fidelity and often, but not always, openness to children. Once ignorance is swept away for a couple who think their second union is truly a marriage, then the divine law of chastity, not the ideal, becomes operative. It may seem impossible for many reasons (children, finances, seeming impossibility to live as brother and sister because of habituation to sexual desire), yet the same Church teaches:
No one should say that the observance of God’s commandments is impossible for the man justified, for, God does not command the impossible, but by commanding He inspires you to do what you can and to ask what you cannot, and He assists you so that you may be able (1 Council of Trent, Decree on Justification, ch. 11, DS 1536; cf.DS 1568-71).
If some authorities of the Church do not believe that heroic virtue is obligatory for repentant grave sinners, then they would necessarily have to deny this teaching of the Tradition going back to St. Augustine or further back in time. It is sometimes true what Pope Francis has said, namely: “Hence it is can no longer simply be said that all those in any “irregular” situation are living in a state of mortal sin and are deprived of sanctifying grace ” (Amoris Laetitia n. 301). However, once ignorance of the sinful situation is removed by truthful instruction on the commandment of God concerning divorce and no remarriage, a couple does not have to be theologically astute to accept it. Rather, they are called to have humble faith and obey God, not a theological opinion, in order for the grace of God to be restored to their souls. Yes, it is clear that they may feel in their consciences that their concrete situation is such that it does not allow them to separate permanently. There may be financial problems which impede their ability to make such a decision. Further they may have serious reasons not to separate because they think they will commit more sin by harming their children. Living as brother and sister may be totally out of the question because they have deep affection for each other. But as Aquinas reminds us: “Essentially…by one sinful act, a man is disposed to commit more readily another like act; because acts cause dispositions and habits inclining to like acts” (ST I-II 88, 4). So, the judgment of conscience or prudence can be easily deceived toward living in a state of sin and the counsel given at the Council of Trent will be foreign or even judged erroneous unless someone is given exceptional graces to assent to the truth and do it. Since the Church teaches no one can possess with the absolute certitude of faith that he or she is personally in the state of grace, it is spiritually dangerous to think “Divorce and remarriage is good and holy for us” (3 DS 1534; cf. St. Thomas on whether someone knows he or she is in the state of grace, ST I-II 112, 5).
This may appear like throwing stones on couples who wish they could completely repent, but these stones are about reality not wishful thinking. We are called to live forever in bliss but we can choose to take a confused road to death forever willfully following an erroneous conscience created by ourselves.