QUESTION: “Does Paul’s prayer for Onesiphorus to find mercy on Judgement Day (2 Timothy 1:18) provide Bible authority for praying for the dead?”
Some have sought to argue this position. Roman Catholic theologians frequently appeal to the text in an attempt to establish their case for praying on behalf of the dead. Regrettably, even some Protestants have yielded to this position, in spite of a total lack of solid evidence for the case, and in spite of evidence which is decidedly against it.
First, the following article, from The Catholic Encyclopedia (online) presents an authoritative position regarding the matter.
“In his Second Epistle to Timothy (i, 16-18; iv, 19) St. Paul speaks of Onesiphorus in a way that seems obviously to imply that the latter was already dead: ‘The Lord give mercy to the house of Onesiphorus’ — as to a family in need of consolation. Then, after mention of loyal services rendered by him to the imprisoned Apostle at Rome, comes the prayer for Onesiphorus himself, ‘The Lord grant unto him to find mercy of the Lord in that day’ (the day of judgment); finally, in the salutation, ‘the household of Onesiphorus’ is mentioned once more, without mention of the man himself. The question is, what had become of him? Was he dead, as one would naturally infer from what St. Paul writes? Or had he for any other cause become separated permanently from his family, so that prayer for them should take account of present needs while prayers for him looked forward to the day of judgment? Or could it be that he was still at Rome when the Apostle wrote, or gone elsewhere for a prolonged absence from home? The first is by far the easiest and most natural hypothesis; and if it be admitted, we have here an instance of prayer by the Apostle for the soul of a deceased benefactor” (http://www.newadvent.org/cathen/04653a.htm).
In 2 Timothy 1, there is a form of prayer on behalf of the family of Onesiphorus (v. 16). Subsequently, in verse 18, the apostle prays for Onesiphorus himself. He petitions the Lord that this brother might “find mercy” in “that day,” which obviously is the day of Judgment.
Because the verbs regarding the brother are all in the past tense, and since only his family is alluded to later in 4:19, some have assumed that Onesiphorus was dead (White, p. 159; Fee, p. 237; Kelly, p. 171). The latter argues that this reflects a New Testament example of prayer on behalf of the dead. N.J.D. White also contended that the apocryphal 2 Maccabees (12:44-45) would allow an orthodox Jew to pray for the dead (p. 159). Fee is more cautious.
In response, this must be noted.
Accordingly, these texts in Paul’s second epistle to Timothy do not come remotely close to providing the coveted evidence by Catholic theologians for the validity of prayers for the dead.
Also, the apostle John teaches us to NOT pray for those who commit a sin unto death (physically). "If anyone sees his brother committing a sin not leading to death [physically], he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death [physically]; I do not say that he should make request for this. All unrighteousness is sin, and there is a sin not leading to death." (1 John 5:16-17). Death is either physical or spiritual. The simplest explanation is that if someone has sinned and died (ex. died from drunkenness, drug overdose, etc.) it is too late to pray for them. Their fate is sealed at death. However, those who have sinned and not yet died physically, still have time to repent. We should pray for those but not those who have sinned and died physically.