The writer informs us himself at the beginning of his treatise that he felt moved by the example of St. Paul, after speaking about virgins, to continue with the subject of widows. But there was also another matter in his own diocese which touched him personally, and caused him at once to take up the matter. A certain widow who had several daughters, some married already and others of marriageable age, began to think of a second marriage for herself. St. Ambrose, partly for her own sake, partly that it might not be supposed that he had in any way advised the step, published the following treatise.
In the first place he affirms that the profession of widowhood comes very close to that of virginity, and is to be esteemed far above the married state. He proves this by the testimony of St. Paul and by his description of one who is a widow indeed; also by many examples taken both from the old and New Testament. Having mentioned St. Peter's wife's mother, he turns more particularly to the widow for whose sake he is writing, though he avoids mentioning her name, pointing out how really empty and insufficient are all the reasons she is setting before herself for marrying again. The marriage bond is, indeed, he says, holy and good, and the married and single are as various kinds of flowers in the field of the church. There is, however, more grain produced than lilies, more that is married than virgin. He points out that widowhood has been held in dishonour by idolaters alone, for which reason it may well be held in honour by Christians. St. Ambrose does not condemn a second marriage, though placing widowhood before it, as being bound to aim at leading those committed to his loving care to the highest possible degree of perfection.
The treatise was written not long after that concerning Virgins, that is, soon after a.d. 377.
Source: Concerning Widows (New Advent)