To quote Dom Guéranger:
Durandus, Bishop of Mende, in his Rational, tells us that this and the following Sundays till Advent bear closely on the Gospel of the marriage-feast, of which they are really but a further development. 'Whereas,' says he, speaking of this twenty-first Sunday, 'this marriage has no more powerful opponent than the envy of satan, the Church speaks to us to-day on our combat with him, and on the armour wherewith we must be clad in order to go through this terrible battle, as we shall see by the Epistle.'
This is the Epistle, taken from St Paul to the Ephesians vi, 10-17:
Brethren, be strengthened in the Lord, and in the might of his power. Put you on the armour of God, that you may be able to stand against the deceits of the devil. For our wrestling is not against flesh and blood; but against principalities and power, against the rulers of the world of this darkness, against the spirits of wickedness in the high places. Therefore take unto you the armour of God, that you may be able to resist in the evil day, and to stand in all things perfect. Stand therefore, having your loins girt about with truth, and having on the breastplate of justice, and your feet shod with the preparation of the gospel of peace: in all things taking the shield of faith, wherewith you may be able to extinguish all the fiery darts of the most wicked one. And take unto you the helmet of salvation, and the sword of the Spirit (which is the word of God).
It seems to me that this passage is most apposite for those going on retreat, and that the very next verse (Eph. vi, 18) is likewise noteworthy:
By all prayer and supplication praying at all times in the spirit; and in the same watching with all instance and supplication for all the saints.
I also note that I have commented on this passage before, since it is also used at Mass for St Anthony the Great.
But to continue quoting Guéranger:
As regards the present Sunday, the portion of the Mass which used formerly to attract the attention of our Catholic forefathers was the Offertory, taken from the Book of Job, with its telling exclamations and its emphatic repetitions. We may, in all truth, say that this Offertory contains the ruling idea which runs through this twenty-first Sunday after Pentecost.
Reduced, like Job on the dung-hill, to the extremity of wretchedness, the world has nothing to trust to but God's mercy....As Amalarius says, the anthem, which has been retained, gives us the words of the historian, who simply relates the facts, one after the other, without any remarks; but, in the verses [previously sung, and still usable pro opportunitate], we have Job himself speaking, his body all humbled, and his soul full of sorrow: the repetition of the same words, their interruptions, their refrain, their broken phrases, vividly represent his panting for breath, and intense suffering.
OFFERTORYVir erat in terra Hus nomine Job, simplex et rectus ac timens Deum [Job i, 1]: quem satan petiit, ut tentaret; et data est ei potestas a Domino in facultates [cf. Job i, 12], et in carnem ejus [cf. Job ii, 5-6], perdiditque omnem substantiam ipsius, et filios [cf. Job i, 13-19]: carnem quoque ejus gravi ulcere vulneravit [cf. Job ii, 7].(There was a man in the land of Hus whose name was Job, simple and upright, and fearing God: and satan asked to tempt him; and power was given him by the Lord over all his possessions, and over his flesh: and he destroyed all his substance, and his sons: and he wounded his flesh with a grievous ulcer.)V/. I. Utinam appenderentur peccata mea; utinam appenderentur peccata mea, quibus iram merui, quibus iram merui; et calamitas, et calamitas quam patior: hæc gravior appareret. [Job vi, 2. 3a](Oh! that my sins were weighed in a balance! Oh! that my sins, whereby I have deserved wrath, whereby I have deserved wrath, were weighed in a balance! and the calamity, the calamity that I suffer, it would appear heavier!)Vir erat...V/. II. Quæ est enim, quæ est enim, quæ est enim fortitudo mea ut sustineam? aut quis finis meus ut patienter agam? [Job vi, 11](For what is, for what is, for what is my strength, that I can hold out? or what is my end, that I should keep patience?)Vir erat...V/. III. Numquid fortitudo lapidum est fortitudo mea? aut caro mea ænea est? ut caro mea ænea est? [Cf. Job vi, 12](Is my strength the strength of stones? or is my flesh of brass? or is my flesh of brass?)Vir erat...V/. IV. Quoniam, quoniam, quoniam non revertetur oculus meus ut videat bona, ut videat bona, ut videat bona, ut videat bona, ut videat bona, ut videat bona,ut videat bona, ut videat bona, ut videat bona. [Job vii, 7b](For, for, for, mine eye shall not return to see good things, to see good things, to see good things, to see good things, to see good things, to see good things, to see good things, to see good things, to see good things.)Vir erat...
This, then, is the celebrated Offertory Vir erat, whose verses in their proper Gregorian setting would tax a soloist with their cascading melismas - see transcription here (and listen!); which, as another writer (Parsch, I think) put it, must have been such a favourite of the Papal choir way back in the early centuries that they simply had to sing it at Sunday Mass once a year, even if, pace the later medieval writers, it doesn't really fit so naturally with all the other chants and lessons and prayers of the Proper of the day.