Philip Schaff (see #9)
2 Timothy 1:16-18 (RSV) May the Lord grant mercy to the household of Onesiph'orus, for he often refreshed me; he was not ashamed of my chains,  but when he arrived in Rome he searched for me eagerly and found me --  may the Lord grant him to find mercy from the Lord on that Day -- and you well know all the service he rendered at Ephesus.
2 Timothy 4:19 Greet Prisca and Aq'uila, and the household of Onesiph'orus.
I have written about this issue in the past; notably in my book, The Catholic Verses, pp. 169-174, and in A Biblical Defense of Catholicism, pp. 141-143.
1) Alfred Plummer (1841-1926) (Anglican): The Expositor's Bible (edited by W. Robertson Nicoll), The Pastoral Epistles, London: Hodder & Stoughton, 1891, pp. 324-326:
Certainly the balance of probability is decidedly in favour of the view that Onesiphorus was already dead when St. Paul wrote these words. . . . he here speaks of "the house of Onesiphorus" in connexion with the present, and of Onesiphorus himself only in connexion with the past. . . . it is not easy to explain this reference in two places to the household of Onesiphorus, if he himself was still alive. In all the other cases the individual and not the household is mentioned. . . . There is also the character of the Apostle's prayer. Why does he confine his desires respecting the requital of Onesiphorus' kindness to the day of judgment? . . . This again is thoroughly intelligible, if Onesiphorus is already dead.2) James Maurice Wilson (1836-1931) (Anglican): Truths New and Old, Westminster: Archibald Constable & Co., 1900, p. 141:
. . . there seems to be equal absence of serious reason for doubting that the words in question constitute a prayer. . . .
Having thus concluded that, according to the more probable and reasonable view, the passage before us contains a prayer offered up by the Apostle on behalf of one who is dead, we seem to have obtained his sanction, and therefore the sanction of Scripture, for using similar prayers ourselves. . . .
This passage may be quoted as reasonable evidence that the death of a person does not extinguish our right or our duty to pray for him: but it ought not be quoted as authority for such prayers on behalf of the dead as are very different in kind from the one of which we have an example here. Many other kinds of intercession for the dead may be reasonable and allowable; but this passage proves no more than that some kinds of intercession for the dead are allowable; viz., those in which we pray that God will have mercy at the day of judgment on those who have done good to us and others, during their life upon earth.
We have, therefore, the sanction of St. Paul for remembering inn our prayers, and interceding for, those who have now passed into the other world . . .3) Sydney Charles Gayford (Anglican): The Future State, New York: Edwin S. Gorham, second edition, 1905, pp. 56-57:
. . . the most satisfactory explanation is that Onesiphorus was dead. . . .4) John Henry Bernard (1860-1927) (Anglican), The Pastoral Epistles, Cambridge University Press, 1899, p. 114:
And so we may hold with some confidence that we have in this passage the authority of an Apostle in praying for the welfare of the departed.
On the whole then it seems probable that Onesiphorus was dead when St. Paul prayed on his behalf . . .5) Donald Guthrie (1915-1992) (Anglican): The Tyndale New Testament Commentaries, The Pastoral Epistles: An Introduction and Commentary, Grand Rapids, Michigan, Eerdmans Pub. Co., 2nd edition, 1990, p. 148:
Since it is assumed by many scholars that Onesiphorus was by now dead, the question has been raised whether this sanctions prayer for the dead. Roman catholic theologians claim that it does. Spicq, for instance, sees here an example of prayer for the dead unique in the New Testament. Some Protestants agree with this judgment and cite the Jewish precedent of 2 Macc 12:43-45 . . .6) William Barclay (1907-1978) (Presbyterian / Church of Scotland), The Letters to Timothy, Titus, and Philemon, Louisville: Westminster John Knox Press, 3rd edition, 2003, p. 175:
. . . there are many who feel that the implication is that Onesiphorus is dead. It is for his family that Paul first prays. Now, if he was dead, this passage shows us Paul praying for the dead, for it shows him praying that Onesiphorus may find mercy on the last day.7) J. N. D. Kelly (1909-1997) (Anglican): A Commentary on the Pastoral Epistles, London: A&C Black, 1963, p. 171:
On the assumption, which must be correct, that Onesiphorus was dead when the words were written, we have here an example, unique in the N.T., of Christian prayer for the departed. . . . the commendation of the dead man to the divine mercy. There is nothing surprising in Paul's use of such a prayer, for intercession for the dead had been sanctioned in Pharisaic circles at any rate since the date of 2 Macc 12:43-45 (middle of first century B.C.?). Inscriptions in the Roman catacombs and elsewhere prove that the practice established itself among Christians from very early times.8) John E. Sanders (evangelical / open theist): No Other Name, Grand Rapids, Michigan: Eerdmans, 1992, pp. 182-183:
Some scholars contend that 2 Timothy 1:16-18 contains a reference to praying for the dead; they contend that the person for whom Paul prays, Onesiphorus was dead.9) Philip Schaff (1819-1893) (Reformed Protestant), The International Illustrated Commentary on the New Testament, New York: Charles Scribner's Sons, 1889, Vol. IV, The Catholic Epistles and Revelation, p. 587:
Footnote 11: Among those commentators who understand Paul to be praying for the dead here are the following: W. Robertson Nicoll, The Expositor's Greek Testament, Vol. 4 (Grand Rapids: William B. Eerdmans, 1951), p. 159; Henry Alford, The Greek Testament, Vol. 3 (Chicago: Moody Pres, 1958), p. 376 . . . J. E. Huther, Critical and Exegetical Handbook to Timothy and Titus (Edinburgh: T. & T. Clark, 1871), p. 263.
On the assumption already mentioned as probable, this would, of course, be a prayer for the dead. The reference ot the great day of judgment falls in with this hypothesis. . . . From the controversial point of view, this may appear to favour the doctrine and practice of the Church of Rome . . .10) Charles John Ellicott (1816-1905) (Anglican): A New Testament Commentary for English Readers, London: Cassell & Co., Vol. III, 1884, p. 223:
There is but little doubt that when St. Paul wrote this Epistle Onesiphorus' death must have recently taken place . . .
The Apostle can never repay now . . . the kindness his dead friend showed him in his hour of need; so he prays that the Judge of quick and dead may remember it in the awful day of judgment. . . .
This passage is famous from its being generally quoted among the very rare statements of the New Testament which seem to bear upon the question of the Romish doctrine of praying for the dead. . . . we here in common with Roman Catholic interpreters and the majority of the later expositors of the Reformed Church, assume that Onesiphorus was dead when St. Paul wrote to Timothy, and that the words used had reference to St. Paul's dead friend . . .