Christ Himself teaches that divorce is forbidden. This means that the bond of a consummated sacramental marriage is not dissoluble except by the death of the spouse. The "porneia" exception clause does not mean one of the spouses cheating during the marriage (Christ did not say "moicheia"), but pertains to some irregularity prior to the marriage which would allow for an annulment.
West: Hermas says in 140 [Shepherd of Hermas 2:4:1:6], "And I said to him [the Angel of Repentance who appeared as a shepherd], 'What then, sir, is the husband to do, if his wife continue in her vicious practices?' And he said, 'The husband should put her away, and remain by himself. But if he put his wife away and marry another, he also commits adultery.'"
WRH: By "divorce," the holy brother of Pope St. Pius I means a separation of spouses, but not dissolution of the marriage bond. For if it were possible for the bond of a consummated sacramental marriage to be dissolved before the death of one of the spouses, it would not be adultery for one of the spouses to remarry.
East: St. Justin Martyr the Philosopher of Caesarea says [Apology 1:15 in PG 6:349AB],
And, "Whosoever shall marry her that is divorced from another husband, commits adultery." And, "There are some who have been made eunuchs of men, and some who were born eunuchs, and some who have made themselves eunuchs for the Kingdom of Heaven's sake; but all cannot receive this saying [Matthew 19:12]." So that all who, by human law, are twice married, are in the eye of our Master sinners, and those who look upon a woman to lust after her. For not only he who in act commits adultery is rejected by Him, but also he who desires to commit adultery: since not only our works, but also our thoughts, are open before God.East: Athenagoras of Athens says in 177 [A Plea for the Christians 33 in PG 6:965A],
For we bestow our attention, not on the study of words, but on the exhibition and teaching of actions,— that a person should either remain as he was born, or be content with one marriage; for a second marriage is only a specious adultery. For whosoever puts away his wife, says He, and marries another, commits adultery [Matthew 19:9]; not permitting a man to send her away whose virginity he has brought to an end, nor to marry again. For he who deprives himself of his first wife, even though she be dead, is a cloaked adulterer, resisting the hand of God, because in the beginning God made one man and one woman, and dissolving the strictest union of flesh with flesh, formed for the intercourse of the race.3rd Century
East: St. Clement of Alexandria says Stromata 2:23 in PG 8:1096B-D,1097A:
Now that the Scripture counsels marriage, and allows no release from the union, is expressly contained in the law,You shall not put away your wife, except for the cause of fornication; and it regards as fornication, the marriage of those separated while the other is alive. Not to deck and adorn herself beyond what is becoming, renders a wife free of calumnious suspicion, while she devotes herself assiduously to prayers and supplications; avoiding frequent departures from the house, and shutting herself up as far as possible from the view of all not related to her, and deeming housekeeping of more consequence than impertinent trifling. He that takes a woman that has been put away, it is said, commits adultery; and if one puts away his wife, he makes her an adulteress, that is, compels her to commit adultery. And not only is he who puts her away guilty of this, but he who takes her, by giving to the woman the opportunity of sinning; for did he not take her, she would return to her husband. What, then, is the law? [Lev 20:10; Dt 22:22] In order to check the impetuosity of the passions, it commands the adulteress to be put to death, on being convicted of this; and if of priestly family, to be committed to the flames [Lev 21:9]. And the adulterer also is stoned to death, but not in the same place, that not even their death may be in common. And the law is not at variance with the Gospel, but agrees with it. How should it be otherwise, one Lord being the author of both? She who has committed fornication lives in sin, and is dead to the commandments; but she who has repented, being as it were born again by the change in her life, has a regeneration of life; the old harlot being dead, and she who has been regenerated by repentance having come back again to life. The Spirit testifies to what has been said by Ezekiel, declaring, I desire not the death of the sinner, but that he should turn [Ek 33:11]. Now they are stoned to death; as through hardness of heart dead to the law which they believed not. But in the case of a priestess the punishment is increased, because to whom much is given, from him shall more be required [Lk 12:48].4th Century
East: Patriarch St. John Chrysostom the Great of Constantinople (Doctor): De libello repud. in PG 51:218:
"What therefore God hath joined together, let not man put asunder." See a teacher's wisdom. I mean, that being asked, Is it lawful? He did not at once say, It is not lawful, lest they should be disturbed and put in disorder, but before the decision by His argument He rendered this manifest, showing that it is itself too the commandment of His Father, and that not in opposition to Moses did He enjoin these things, but in full agreement with him. But mark Him arguing strongly not from the creation only, but also from His command. For He said not, that He made one man and one woman only, but that He also gave this command that the one man should be joined to the one woman. But if it had been His will that he should put this one away, and bring in another, when He had made one man, He would have formed many Women. But now both by the manner of the creation, and by the manner of lawgiving, He showed that one man must dwell with one woman continually, and never break off from her.WRH: One of the greatest of the Eastern doctors clearly states that divorce and subsequent remarriage are contrary to the design of God Almighty.
West: Pope St. Innocent I of Rome says in 408 [Epist. ad Exsuper n. 12 in PL 20:500]: "The practice is observed by all of regarding as an adulteress a woman who marries a second time while her husband yet lives, and permission to do penance is not granted her until one of them is dead."
WRH: This practice, according to the knowledgeable and saintly supreme pastor of the Church, is universal. This means that in the fifth century the undivided Church regarded the marriage bond to be indissoluble.