Paul asked for God’s mercy on Onesiphorus because of the help he HAD been to Paul’s ministry, before Onesiphorus abandoned the Gospel. Onesiphorus had begun the race well, but apparently did not finish well.
Let’s review the passage one more time, with an addition:
The Lord grant mercy to the household of Onesiphorus, for he often refreshed me, and was not ashamed of my chain; but when he arrived in Rome, he sought me out very zealously and found me. The Lord grant to him that he may find mercy from the Lord in that Day– and you know very well how many ways he ministered to me at Ephesus.
You therefore, my son, be strong in the grace that is in Christ Jesus.
It is my opinion that the ‘You therefore’ or ‘Then you’ (depending upon the translation) refers back to the discourse on those who have not been faithful – and those that have.
Further, I believe that we can connect Onesiphorus to those unashamed of Paul:
Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God, (2Ti 1:8 NKJ)
For this reason I also suffer these things; nevertheless I am not ashamed, for I know whom I have believed and am persuaded that He is able to keep what I have committed to Him until that Day. (2Ti 1:12 NKJ)
The Lord grant mercy to the household of Onesiphorus, for he often refreshed me, and was not ashamed of my chain; (2Ti 1:16 NKJ)
Paul speaks of the shame of being bound in chains (he would have been chained to a Roman solider for most of the day), but Paul was not ashamed as he had committed his soul to God already, for That Day. The same idea, unashamed because of commitment on That Day, is turned upon Onesiphorus.
Please allow me to clarify: I do not believe that this passage, or any passage, supports prayer for the dead. This has been an exercise in dialog. There are just continuing thoughts in a good conversation.
It is worth noting that we are not the only ones concerned with Onesiphorus – a 2nd Century work purported to report on the events of Paul and some of his interaction with the man and his family.