I believe in the Communion of Saints- this statement is taken from the Apostle's Creed. All Christians believe, or should believe in the Communion of Saints. But some Christians do not understand why Catholic Christians (Catholics, Orthodox, Anglo-Catholics) pray to the saints, and pray for the dead.
The Communion of Saints is based on the Holy Eucharist:
Because there is one loaf, we, who are many, are one body, for we all partake of the one loaf. - 1 Corinthians 10:17 NIV
Death cannot break this bond of communion we have with Christ and each other. Every time we partake of the Holy Eucharist, we are celebrating it not only with those believers in our particular Church on a Sunday morning, but with every Christian who has ever celebrated and partaken in the Holy Eucharist:
But you have come to Mount Zion, to the heavenly Jerusalem, the city of the living God. You have come to thousands upon thousands of angels in joyful assembly, to the church of the firstborn, whose names are written in heaven. You have come to God, the judge of all men, to the spirits of righteous men made perfect, to Jesus the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel. - Hebrews 12:22-24 NIV
In coming to Christ, we do not come as an individual. We come as a community, as a body. As individuals, when we come to Christ, we come also to the assembly of angels and saints, in one body. The Sprinkled blood mentioned in this passage is the very Blood of Christ, shared in the Holy Sacrifice of the Eucharist.
Because of this solidarity of one Body of Christ that unites heaven and earth, we can pray for those who have died before us, and they can pray for us.
We believe we can pray for those who have gone before us, "in faith and hope of the resurrection," in the words of one Eastern Orthodox prayer.
He then took up a collection among all his soldiers, amounting to two thousand silver drachmas, which he sent to Jerusalem to provide for an expiatory sacrifice. In doing this he acted in a very excellent and noble way, inasmuch as he had the resurrection of the dead in view; for if he were not expecting the fallen to rise again, it would have been useless and foolish to pray for them in death. But if he did this with a view to the splendid reward that awaits those who had gone to rest in godliness, it was a holy and pious thought. Thus he made atonement for the dead that they might be freed from this sin. - 2 Maccabees 12:43-46 NAB
Here is the commentary from the New American Bible on this passage: "This is the earliest statement of the doctrine that prayers (2 Macc 12:42) and sacrifices (2 Macc 12:43) for the dead are efficacious. The statement is made here, however, only for the purpose of proving that Judas believed in the resurrection of the just (2 Macc 7:9, 14, 23, 36). That is, he believed that expiation could be made for certain sins of otherwise good men-soldiers who had given their lives for God's cause. Thus, they could share in the resurrection. His belief was similar to, but not quite the same as, the Catholic doctrine of purgatory."
Likewise, we believe that the Saints in heaven can pray for us Saints here on earth as well. As the passage from Hebrews above indicates, we have come to the "spirits of just men made perfect." This belief antedates the Christian era, as we see in this passage from 2 Maccabees:
[Judas Maccabeus] cheered them all by relating a dream, a kind of vision, worthy of belief. What he saw was this: Onias, the former high priest, a good and virtuous man, modest in appearance, gentle in manners, distinguished in speech, and trained from childhood in every virtuous practice, was praying with outstretched arms for the whole Jewish community. Then in the same way another man appeared, distinguished by his white hair and dignity, and with an air about him of extraordinary, majestic authority. Onias then said of him, "This is God's prophet Jeremiah, who loves his brethren and fervently prays for his people and their holy city." Stretching out his right hand, Jeremiah presented a gold sword to Judas. As he gave it to him he said, "Accept this holy sword as a gift from God; with it you shall crush your adversaries." - 2 Maccabees 15:11-16 NAB.
Here is commentary from the New American Bible on this passage: "Onias, the former high priest: Onias III (2 Macc 3:1-40). Evidently the author believed that the departed just were in some way alive even before the resurrection." Jeremiah: regarded by the postexilic Jews as one of the greatest figures in their history; cf 2 Macc 2:1; Matthew 16:14. Who . . . prays for his people: a clear belief in the intercession of the saints.
We can pray for those who gone before us, and they can pray for us. There is solidarity of love in the One Body of Christ. Death cannot break that bond. We acknowledge this reality every time we celebrate the Divine Liturgy, the Holy Eucharist in our Churches. Consider this passage from the Book of Revelation:
When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained. They called out in a loud voice, "How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?" Then each of them was given a white robe, and they were told to wait a little longer, until the number of their fellow servants and brothers who were to be killed as they had been was completed. - Revelation 6:9-11 NIV
The Orthodox Study Bible offers this commentary on the passage: the vision of the souls of the martyrs under the altar (v.9) is derived from the OT practice of pouring blood (the physical manifestation of the life of the soul) of sin offering at the base of the altar of burnt offering. It is a basis for the historic Christian practices of building church buildings over the tombs of martyrs, placing relics of saints in the altar when a church is consecrated, and burying baptized people under the altar. Thus in the Divine Liturgy Orthodox Christians remember "those who lie asleep here, and in all the world."
The Bible says, "The memory of the just will be blessed." - Proverbs 10:7 NAB.
We honor the saints because they are blessed. Mary the Mother of God prophesies of herself, "For behold, henceforth all generations will call me blessed."- Luke 1:48, NKJV
The Saints have come to the glory that awaits us all. For Catholic Christians, death is a sacrament, a passage way on our path to deification and glorification with the Saints with God in heaven.