At Jimmy Akin’s blog some great posts
jimmyakin.typepad.com/defensor_fidei/2008/08/prayer-for-the.html
…As Jimmy pointed out, even if you don’t accept that Maccabees is canonical, at the very least you have to admit that it is an authentic text dating to about 100 years before the time of Christ, and that therefore it presents evidence that prayer for the dead was a venerable custom already present when Jesus walked the earth. If it were not true, don’t you think that Jesus would have spoken against it?
…Whether you accept the canonicity of Second Maccabees or not, the book shows that the belief that prayer is profitable for the dead already exist among the Jews in the second century before Christ. That the Jews continue to accept this belief in first centuries of Christian era is evident from certain ancient Jewish works, like 'Sifre to Deuteronomy[a Midrash]'and ‘The Testament of Abraham[First or second century AD]’,which speak of suffrages for the dead. Therefore, Christ and the Apostles, who were Jews, must have been familiar with the belief that prayer is profitable for the dead. But did they accept it? This is where the historical testimony for the Church’s practice of prayer for the dead comes in. If it can proved historically that prayer for the dead was practiced and accepted in the Church in the earliest days of Christianity, then it must be accepted that Christ and the Apostles accepted that practice because the early Church could not have adopted that practice from Judaism through the pharisees or rabbis. No, the only way the early Church could have carried over that practice from Judaism would be if her founder Jesus Christ and the Apostles, who as Jews were familiar with that practiced, accepted it. After all, this was the only way the early Church carried over several other beliefs and practices from Judaism which the majority of protestants today still accept with us (i.e reality of hell, resurrection of the dead, immortality of the soul, e.t.c).Now, can it be proved historically that prayer for the dead was practiced and accepted in the Church in the earliest days of Christianity? Yes, it can. Check out the following testimonies from ancient Christian literature which dates from the second to fifth century AD:
St.EPIPHANIUS OF SALAMIS(310/320-403):
“Useful too is the prayer fashioned on their behalf…it is useful, because in this world we often stumble either voluntarily or involuntarily.”[Panarion,75:8]
St.AMBROSE OF MILAN (340-397):
“Give, Oh Lord, rest to Thy servant Theodosius, that rest Thou hast prepared for Thy saints…I love him, therefore will I follow him to the land of the living; I will not leave him till by my prayers and lamentations he shall be admitted unto the holy mount of the Lord, to which his deserts call him.”[De obitu Theodosii]
St. JOHN CHRYSOSTOM (347-407):
By praying and entreating others to make prayers for them, by continually giving to the poor on their behalf."[Homilies on Phillipians,3]
St. AUGUSTINE OF HIPPO (354-430):
For some of the dead, indeed, the prayer of the Church or of pious individuals is heard;
c.f. also [edited to be briefer I can supply the quotes to the *c.f. if you wish]