A – Thanks for the great question! There are several reasons that Catholics believe that the Saints in heaven can hear our requests for their intercession. First, let us start with the Biblical evidence.
“And concerning the resurrection of the dead, have you not read what was said to you by God, `I am the God of Abraham, the God of Isaac, and the God of Jacob'? He is not the God of the dead but of the living.” --Matthew 22:31-32----Note that Jesus is telling us that those in heaven are alive. But, they now have a new and higher way of living. They have been glorified in Christ once they enter into heaven. In fact, it could be said that they are much more alive than those of us still on earth.
“And behold, Moses and Elijah appeared to them, conversing with him. Then Peter said to Jesus in reply, "Lord, it is good that we are here. If you wish, I will make three tents here, one for you, one for Moses, and one for Elijah." While he was still speaking, behold, a bright cloud cast a shadow over them, then from the cloud came a voice that said, "This is my beloved Son, with whom I am well pleased; listen to him.” --Matthew 17:3-5----Here, during the Transfiguration, Jesus talks to Moses and Elijah, who are very aware of what has been happening on earth. So, from this we can come to the conclusion that death does not separate those in heaven from those on earth.
Remember Paul teaches that we are all members of Christ’s body, the Church.
The Book of Hebrews echoes this when it teaches that those who have gone before us into heaven still witness what happens on earth.
“Therefore, since we are surrounded by so great a cloud of witnesses, let us rid ourselves of every burden and sin that clings to us and persevere in running the race that lies before us” --Hebrews 12:1
What I believe is the most amazing evidence from the Bible of the Saints in heaven hearing our prayers is from the book of Revelation.
“When he took it, the four living creatures and the twenty-four elders fell down before the Lamb. Each of the elders held a harp and gold bowls filled with incense, which are the prayers of the holy ones.” --Revelations 5:8----We see that the elders and four living creatures (who represent the Saints and Angels in heaven) are offering the prayers of those on earth before Jesus. I don’t think it could get much clearer. In Revelation 8 there is another incident of heavenly intercession.
“Another angel came and stood at the altar, holding a gold censer. He was given a great quantity of incense to offer, along with the prayers of all the holy ones, on the gold altar that was before the throne. The smoke of the incense along with the prayers of the holy ones went up before God from the hand of the angel. Then the angel took the censer, filled it with burning coals from the altar, and hurled it down to the earth. There were peals of thunder, rumblings, flashes of lightning, and an earthquake.” --Revelations 8:3-5
Lastly, we have evidence from Christ himself.
“I tell you, in just the same way there will be more joy in heaven over one sinner who repents than over ninety-nine righteous people who have no need of repentance. "Or what woman having ten coins and losing one would not light a lamp and sweep the house, searching carefully until she finds it? And when she does find it, she calls together her friends and neighbors and says to them, `Rejoice with me because I have found the coin that I lost.' In just the same way, I tell you, there will be rejoicing among the angels of God over one sinner who repents.” --Luke 15:7-10----Those in heaven could not rejoice over a sinner repenting on earth unless they knew about it.
One more reference of Christ talking about this subject is found in the parable of Lazarus and the rich man in Luke 16. Those who have suffered bodily death, still are asking for help for those on earth (intercession) with knowledge of what is happening.
Getting to the question outside the Biblical evidence, we can support the fact that those in heaven can hear our prayers because it fits with what we know about God and humans. Just as no person can achieve heaven on their own power, so a Saint in heaven cannot hear prayers of those on earth from their own power. But, being glorified in Christ they now are partakers of the divine nature. This means they participate in the grace of God to a greater degree than we can even imagine. While God is the only one who by nature is omnipotent, omniscient and omnipresent – the Saints in heaven can in some way share in these traits. The number of prayers offered is finite, so to be able to “hear” all prayers wouldn’t take the gift of omniscience, but rather just raising our nature to a higher level.
We must also remember that the heavenly existence is no longer bound by time. There is no time but eternity in heaven. Therefore, we must not try and answer a question of this nature by using our own limited understanding of how things work in this life.
Based on the overwhelming evidence from Scripture, the constant Tradition of the Church as well as the fact that it is theologically acceptable, we can be assured that the Saints in heaven can hear our prayers and are praying for us.
I hope this answers your question.