What is the Biblical argument used by Evangelical Christians against the Catholic doctrine of the invocation of the saints? The Catholic Church holds to the doctrine of Intercession of the Saints. As I understand it, this refers to the saints in heaven praying for us saints on earth.However, Catholicism teaches that we need to ask the saints in heaven for their prayer. This is the doctrine of the Invocation of the Saints.


  The Catholic doctrine of intercession and invocation is set
  forth by the Council of Trent, which teaches that "...the saints who
  reign together with Christ offer up their own prayers to God for men.
  It is good and useful suppliantly to invoke them, and to have recourse
  to their prayers, aid, and help for obtaining benefits from God,
  through His Son Jesus Christ our Lord, Who alone is our Redeemer and
  Saviour."(source)


Evangelical Christians generally reject this doctrine. So, what is the specific biblical argument they use to oppose the Catholic doctrine of the invocation of the saints?
There are two main Protestant arguments against asking the deceased saints to intercede for us:


It is seen as contradicting the Bible's prohibition on contacting the dead. These are found most clearly in the Jewish Law, which is not binding on Christians, but is still considered to be solid wisdom in this case.


  Do not turn to mediums or seek out spiritists, for you will be defiled by them. I am the Lord your God. (Leviticus 19:31, NIV)
  
  Let no one be found among you who sacrifices their son or daughter in the fire, who practices divination or sorcery, interprets omens, engages in witchcraft, or casts spells, or who is a medium or spiritist or who consults the dead. (Deuteronomy 8:10-11)

It is thought that asking the saints to intercede for us is at best a waste of time, and at worst idolatrous, because Jesus is the best and perfect mediator, and no human can remotely compare to him. And while not all who do believe in the intercession of the saints think this way, there are many people who think that we are too sinful, too unworthy to directly approach Jesus in prayer, and for our requests to be heard we must go through Mary or the saints. Evangelical Protestants would consider this to be a gross distortion of the gospel.


  For there is one God and one mediator between God and mankind, the man Christ Jesus (2 Timothy 4:5)
  
  Now there have been many of those priests, since death prevented them from continuing in office; but because Jesus lives forever, he has a permanent priesthood. Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them. (Hebrews 7:23-25)


This argument may not be very strong, because Protestants don't hesitate to ask living Christians to pray for them, but it's still an argument that gets made.
A third, more minor, argument is an argument from silence: this practice is not recognised as being taught or performed anywhere in the Bible.


This position ultimately arises from a different understanding of the Communion of Saints in Protestantism. Protestants would not deny the real union of living believers (the church militant) nor the union of believers who have passed away (the church triumphant), but they would generally not say that there is a functional union between those groups. We are united in our shared salvation state, our faith, our creed, our shared indwelling of the Spirit, our hope, and our inheritance, but we are not united with the dead in any relational capacity. There is much disagreement within Protestantism over the nature of the Intermediate State between death and resurrection, and many believe in soul sleep, that the dead are unconscious until the time of their resurrection. Others may believe that they are conscious, but that they do not have the means to hear our invocations and prayers.