This was a question that I started asking early on in my research of Catholicism. I understood the basic reasons why Catholics venerate the Saints and Mary, and why they believed they could be asked for prayer. (The actual process of veneration of the Saints, which involves scientifically verifying that a miracle has occurred because of the intercession of a possible Saint, was a key point here.) Being rather skeptical by nature, I decided to research the topic from the other perspective: why can’t we pray to the Saints?
(Spoiler: we can, as far as I can tell. I haven't yet found a reason why we shouldn't.)
Along with doing research of various sources, I posted in several non-denominational online forums, and was met with an interesting array of responses. They ranged from some people saying, “Our denomination says it’s okay; you can pray to the Saints!” to others that said, “It’s not encouraged within the church, but if you were to ask, you’d be told it’s okay”, to others that said, and I quote, “You need to repent from your evil catholic [sic] ways!” (This last one is especially funny because I opened the question stating that I am Protestant.)
Of course, the internet isn’t the greatest place to get in an argument, but it worked for my purposes. A few hundred comments later, I had built a list of reasons why we can’t pray to the Saints. Here they are, in more or less an order that makes sense.
Christ is our mediator
The common Bible verse used here is 1 Timothy 2:5 (“For there is one God and one mediator between God and mankind, the man Christ Jesus”), which makes sense. My issue with this argument is that it doesn’t say anything about asking someone to pray for you. It’s not saying I can’t ask Mary for prayer, nor is it saying that I can’t ask a friend for prayer. As much as this one is touted and explained, I really don’t understand how it’s relevant.
The dead aren't omniscient
Many people argued that the dead can’t hear our prayers, but gave no solid reason why. There’s also the question of if we are saved, don’t we have everlasting life in Christ? So, by that argument, the Saints aren’t dead at all.
There’s also the whole discussion of the fact we don’t really have much of an idea what goes on after bodily death. (Ignoring the Catholic stance on this) we really don’t know if they can hear our prayers or not.
The Bible doesn't say we can
The Bible doesn’t say we can do lots of things. It doesn’t say we can celebrate the birth of Jesus, so does that mean we aren’t allowed to have Christmas? Uh oh.
This one really boils down to the whole sola scriptura aspect of Protestantism, and it's whole denial of the authority of the Church. If the Bible doesn't teach on a specific topic that has newly arisen, how are we to know what is right and wrong?
“It’s pointless. You have direct authority to talk to God because of Jesus, why ask someone else assuming that they are actually around to listen to you?”
Again, why can’t I? If I’m in a predicament, wouldn’t I want to get as many people praying for me as possible?
Abomination of necromancy (Deuteronomy 18:10-12)
This verse is instruction that we are not to conjure the dead to gain information, but to look to God's prophets instead. There's no way that having a seance to attempt to have the dead speak through you or a medium could be placed in the same group as someone who just wants to say, "St Joseph, pray for me!".
Divination is forbidden (Leviticus 19:31 & Leviticus 20:6)
Both of these verses are talking about divination, which is attempting to contact a the dead or a supernatural being for secret powers or information through a medium, which is different than asking someone for prayer. There's also the question of who or what are the beings being contacted? Many would argue that it’s not the dead, but demons or Satan masquerading as the dead in order to gain power.
Praying to anything that is not God is idolatry
This is my most despised of the arguments. It’s just semantics. “Pray” does not equal “worship”. Idolatry is worshiping, not praying to. Sure, it’s an uncommon use of the word, but I can pray to my Mom that she’ll make me my favourite food when I visit, and there’s no linguistic reason which says that means I’m worshipping her.
That’s the main reasons that I was given that we can’t pray to the Saints. I’m convinced that we can ask Mary and the Saints for prayer (my wife did buy me a rosary recently, after all), so this discussion had fallen a bit to the back burner, but I felt I should write this up so as to not forget about it all together. If you have a reason that’s not mentioned, and you’re willing to explain it to me fully (unlike some Redditors), do get in touch.