12 But thou shouldest not have looked on the day of thy brother in the day that he became a stranger; neither shouldest thou have rejoiced over the children of Judah in the day of their destruction; neither shouldest thou have spoken proudly in the day of distress.
[AD 397] Ambrose of Milan on Obadiah 1:12
Preserve, O Lord, Thy work, guard the gift which Thou hast given even to him who shrank from it. For I knew that I was not worthy to be called a bishop, because I had devoted myself to this world, but by Thy grace I am what I am. And I am indeed the least of all bishops, and the lowest in merit; yet since I too have undertaken some labour for Thy holy Church, watch over this fruit, and let not him whom when lost Thou didst call to the priesthood, to be lost when a priest. And first grant that I may know how with inmost affection to mourn with those who sin; for this is a very great virtue, since it is written: "And thou shall not rejoice over the children of Judah in the day of their destruction, and speak not proudly in the day of their trouble." Grant that so often as the sin of any one who has fallen is made known to me I may suffer with him, and not chide him proudly, but mourn and weep, so that weeping over another I may mourn for myself, saying, "Tamar hath been more righteous than I."

[AD 420] Jerome on Obadiah 1:12
(Verse 12.) And do not despise the day of your brother in the day of his journey. And do not rejoice over the children of Judah in the day of their destruction, and do not boast in the day of distress; nor enter the gate of my people in the day of their ruin. LXX: And do not despise the day of your brother in the day of strangers. And do not rejoice over the children of Judah in the day of their destruction; and do not speak proudly in the day of distress; nor enter the gates of the nations in the day of their labors. The same sentiment as above: When the man from Mount Esau is destroyed, on account of his killing and wickedness towards his brother Jacob, and eternal confusion covers him, he will by no means do what he did before against his brother; for he will not despise or scorn, seeing his brother go into captivity; nor will he rejoice over the sons of Judah. Indeed, two tribes, except for the Levitical, which reigned in Jerusalem and were called Judah, were captured by the Chaldeans. And you will not boast greatly, and thinking yourself to be one of the victors, you will laugh at the distress of your brother. Nor on the day of the devastation and ruin of the people, will you walk through the gate of Jerusalem with glory. And for this reason, you will not do these things, because you will suffer similar. Otherwise, when you see your brother taken captive by different persecutions, and being led away bound from the faith of the Church, not pursuing his own city, but all foreign things, you will not rejoice at all, because you will suffer similar. For you rejoiced when Jacob was captured and when the sons of Judah, whom we understand to be the disciples of Christ, were brought low on the day of their destruction. It is signified, however, that the soul is placed in the midst of vices and virtues, and can be turned in either direction in the space of a moment. 'You shall not magnify your mouth on the day of distress,' he says. This we should understand in two senses: the bodily distress in persecutions and sins, and the spiritual distress when the soul, captured by its enemies and vices, is led into Babylon. Nor shall you enter the gate of my people on the day of their ruin: for when either denial or pleasure oppresses us, and an unfortunate conscience does not preserve the former rigor, then we easily slip into contrary doctrines, flattering our error and not caring for the wound, but touching it. It is a solace of misery, although lost, to have something to hope for.

[AD 458] Theodoret of Cyrus on Obadiah 1:12
“Do not despise the day of your brother in the day of his alienation.” Here the time of punishment is called a “day.”

[AD 1781] Richard Challoner on Obadiah 1:12
Thou shalt not look: or, thou shouldst not, etc. It is a reprehension for what they had done, and at the same time a declaration that these things should not pass unpunished.

Thou shalt not magnify thy mouth: That is, thou shalt not speak arrogantly against the children of Juda as insulting them in their distress.