What if every Christian in the world prayed today to St Francis? Would St Francis be able to comprehend those prayers from billions?
It assumes a level of omniscience on the part of a saint that is normally reserved for God. Therefore, you must come up with a teaching on how St Francis comes to know about all these prayers.
If he does it through his own abilities, then he is omniscient.
If he gets it passed from God, then you must wonder why God already knows the request but requires St Francis to remind Him.
I say this politely. My intent is to point out a weakness in the position.
We agree that the Angels are spirits created by God (unlike Adam and Eve, they have no physical bodies). Angels can basically go anywhere and do anything, but they can not "read minds" so to speak; they do not know what is in our hearts. Only God does. That is why the fallen angels -- (angels, like Adam and Eve, were created with free will to accept or reject God) -- in particular Lucifer (Satan) can not read our mind and know our heart. Satan understands human nature like an expert shrink -- he tempts and thwarts in order to cause people to fall from grace, to sin against God. But He can't MAKE someone sin -- we CHOOSE to win, and Satan knows how to tempt humans. As St. Paul said, "12 Therefore, brethren, we are debtors, not to the flesh to live according to the flesh. 13 For if you live according to the flesh, you shall die: but if by the Spirit you mortify the deeds of the flesh, you shall live. 14 For whosoever are led by the Spirit of God, they are the sons of God." (Romans 8:12-14)
OK, I'm assuming that we're still on the same page. If we die in a state of Grace (saved), we go to heaven. We become saints. As saints, we praise and worship God.
If a saint is praying in heaven, why can't he pray for those still on earth? When you go to heaven, to you forget your loved ones? Your friends? Your neighbors? Couldn't a saint pray to God for the conversion of a sinful friend?
Catholics pray directly to God. We also ask the saints to pray for us. So really, we're praying to God and the saints are praying for us, and if the prayer is something the God wishes to grant us, He will.
I suppose one DOESN'T have to ask the saints to pray for us, but there is absolutely no harm. And I believe strongly in the power of prayer. When a group of people get together to pray for someone who is sick, or going through difficult times, I believe that God really listens -- we're praying for our neighbor and showing the love we are commanded to demonstrate as Christians, and I think that God blesses that.
The Catechism of the Catholic Church teaches:
956 The intercession of the saints. "Being more closely united to Christ, those who dwell in heaven fix the whole Church more firmly in holiness.... They do not cease to intercede with the Father for us, as they proffer the merits which they acquired on earth through the one mediator between God and men, Christ Jesus.... So by their fraternal concern is our weakness greatly helped."[493] Do not weep, for I shall be more useful to you after my death and I shall help you then more effectively than during my life.[494] I want to spend my heaven in doing good on earth.[495]God bless.
You assume or imply that it is a saint's own abilities. The Church has never declared such a thing. If in the Kingdom we will "be like Him" it is only because of His Omnipotent Love and Generosity. We won't be infinite like God, but "Eye hath not seen, nor ear heard, neither hath it entered into the heart of man." You doubt God's power to endow His elect with whatever pleases Him?
If he gets it passed from God, then you must wonder why God already knows the request but requires St Francis to remind Him.
Why should I wonder? We can add nothing to God! Yet, in all His Being, He saw it Good to create. True pleasure is derived from good. If God is the GOOD, God must derive pleasure, in this case from His creation. i.e. when Paul said to the Galatians, "But when it pleased God, who separated me from my mother's womb, ...."
No, God doesn't REQUIRE his reminding! That's foolish! God created us in His image and likeness. God only knows why he wants us to participate in His divine life for all eternity.