Here is the preamble to the Panachida in the 'Come to Me' prayerbook ACROD 1985,1995,
"Truly truly, I say to you, he who hear MY word and believes Him Who sent ME, has enternal life; he does not come into judgment but has passed from death to life. (John 5:24).
A Russian writer of the last century, Alexei Khomiakov, in speaking of the meaning of the Church, wrote: 'No one is saved alone. He who is saved is saved in the Church, as a member of her and in union with her all her other members. If anyone believes, he is in the communion of faith; if he loves, he is in the communion of love; if he prays, he is in the communion of prayer.'
When a member of the Orthodox Church 'falls asleep in the Lord', he or she does not cease to be a member of the communion of faith, love and prayer which is the Church. The Church always prays for all her members, even those departed, those awaiting the hour when "we who are alive, who are left, shall be caught up together with them in the clouds to meet the Lord in the air; and so we shall always be with the Lord.' (1 Thessalonians 4:17)
The earliest reference to the prayer for the dead is found in the Second Book of Maccabees, 12:39-45. This shows that the practice was known to the Jews in the centuries just before the time of our Lord Jesus Christ.
The Church has always prayed for the dead at the celebration of the Divine Liturgy. In addition, there is a service of prayer for the dead known as 'The Panachida.' The word 'Pnanchida' comes from the Greek word meaning 'all night'. [sic.] In ancient times it was the custom to spend the night before a funeral in an all-night vigil before the celebration of the Divine Liturgy and the burial service. The current 'Panachida' is taken from that vigil.
The prayer of the priest, 'O God of spirits and of all flesh' dates back to the fourth century and has been in widespread use in the Church since that time. the Panachida may be celebrated after the Divine Liturgy or by itself." pg 125 ibid.