Both the Orthodox church and the Roman Catholic Church do practice the invocation of the saints in heaven, that is, they pray to holy persons ( both to angels and to humans in heaven) who are made holy by God. According to these churches, when we pray to God, we ask him to give us something because God is able to do so but when we pray to a saint to give us something, then, this is impossible because only God answers prayers.This practice only denotes intercession which means that a person only prays to a saint in heaven to pray for him/her.Nothing more, nothing less.
The intercession of the saints in heaven is Biblical but the invocation of the saints in heaven is not biblical based on the facts written in the inspired scriptures:
Intercession of the saints in heaven- this denotes that alive people in Christ who are in heaven intercede through prayer for those living on earth. The Bible is explicit that intercession of the saints in heaven is a teaching of the earliest church, that is, the first century church:
Revelations 8:4: "And the smoke of the incense of the prayers of the
saints ascended up before God from the hand of the angel".
In fact, the Old Testament concurs:
If there shall be an angel speaking for him . . . He shall have mercy
on him, and shall say: Deliver him, that he may not go down to
corruption" (Job 33:23).
And the non-inspired Jewish literature also agrees:
When thou didst pray with tears… I (Archangel Raphael) offered thy
prayer to the Lord. (Tobit 7:12)
The Bible and the Apocrypha both show that people in heaven already pray for people on earth without these people on earth asking for it.
Invocation of the saints in heaven- this denotes that people on earth can ask people in heaven for prayers.
Roman Catholics ask Mary with a prayer like this:
" Holy Mary, Mother of God, pray for us sinners, now and at the hour of
our death. Amen"
But neither the Bible nor the Apocrypha shows that invocation of the saints is an accepted ancient doctrine of either Judaism or the 1st Century church but rather, both the Bible and the Apocrypha clearly shows that intercession of the saints is a reality.
Christians must only pray to the Trinity because the Holy Scriptures only commands us to pray to God. The Scriptures never commanded us to pray to Christians in heaven to pray for us because they're already doing that without us asking them.