When did the Catholic Church first prohibit bishops from remarrying?

When did the Catholic Church first prohibit bishops from remarrying?

For St. Paul being married to a woman for the first time seems to be a prerequisite for the office of the episcopate. Widowers who were remarried would be discouraged from becoming a bishop. Thus the phrase "the husband of one wife” implies that the candidate for this office has never remarried and is either a unmarried widower or married with his first and only wife.

The faithful in the general population have always been free to remarry after the death of their spouses.

1 It is well said, When a man aspires to a bishopric, it is no mean employment that he covets. 2 The man who is to be a bishop, then, must be one with whom no fault can be found; faithful to one wife, sober, discreet, modest, well behaved, hospitable, experienced in teaching, 3 no lover of wine or of brawling, courteous, neither quarrelsome nor grasping. 4 He must be one who is a good head to his own family, and keeps his children in order by winning their full respect; 5 if a man has not learned how to manage his own household, will he know how to govern God’s church? 6 He must not be a new convert, or he may be carried away by vanity, and incur Satan’s doom. 7 He must bear a good character, too, in the world’s eyes; or he may fall into disrepute, and become a prey to the False Accuser. - 1 Timothy 3:1-7

The exhortation of St. Paul in 1 Timothy 3: 1-3 has other interpretations!

The Catholic Encyclopedia infers a rather different approach to this question as follows:

Turning now to the historical development of the present law of celibacy, we must necessarily begin with St. Paul's direction (1 Timothy 3:2, 12, and Titus 1:6) that a bishop or a deacon should be "the husband of one wife". These passages seem fatal to any contention that celibacy was made obligatory upon the clergy from the beginning, but on the other hand, the Apostle's desire that other men might be as himself (1 Corinthians 7:7-8), already quoted) precludes the inference that he wished all ministers of the Gospel to be married. The words beyond doubt mean that the fitting candidate was a man, who, amongst other qualities which St. Paul enunciates as likely to make his authority respected, possessed also such stability of divorce, by remaining faithful to one wife. The direction is therefore restrictive, no injunctive; it excludes men who have married more than once, but it does not impose marriage as a necessary condition. This freedom of choice seems to have lasted during the whole of what we may call, with Vacandard, the first period of the Church's legislation, i.e. down to about the time of Constantine and the Council of Nicaea.

When did the Catholic Church first prohibit bishops from remarrying?

Possibly in 314, we see the first inklings of the notion that the clergy should not remarry after their ordination. What is written for priests and deacons, should apply to higher clergy also.

This is again what we learn from the Council of Ancyra in Galatia, in 314 (canon x), and of Neo-Caesarea in Cappadocia, in 315 (canon i). The latter canon absolutely forbids a priest to contract a new marriage under the pain of deposition; the former forbids even a deacon to contract marriage, if at the moment of his ordination he made no reservation as to celibacy. Supposing, however, that he protested at the time that a celibate life was above his strength, the decrees of Ancyra allow him to marry subsequently, as having tacitly received the permission of the ordaining bishop. There is nothing here which of itself forbids even a bishop to retain his wife, if he were married before ordination. - Celibacy of the Clergy (Catholic Encyclopaedia)

In 1075, Pope Gregory VII made clerical celibacy a mandatory discipline within the Church.

The last pope to be married and pope at the same time was Pope John XVII (1003).

In 1075 Pope Gregory VII issued a decree effectively barring married priests from ministry, a discipline formalized by the First Lateran Council in 1123. Since then celibacy has been required of Roman Catholic priests, though the Catholic churches of the East have continued to allow priests to marry before their ordination. - Why are priests celibate?

Thus, if Pope Gregory VII barred priests from being married, it stands to reason that the higher clergy must also be celebrate, bishops and popes included.

Widowers have always been permitted to become priests and bishops.