Bible
Should Christians pray to Mary or the saints?
Catholic and Orthodox Christians do this; most Protestants don't—and the disagreement comes down to two different ways of reading what's actually happening.
Catholics and Orthodox see it as asking a Christian friend to pray for you, except this friend is already in God's presence in heaven. They point to passages describing believers, living and dead, as one connected family, and carefully distinguish this practice from worship, which they teach is reserved for God alone.
Most Protestants reject the practice, even while agreeing prayer for others is beautiful. Their concerns: Scripture never actually shows anyone praying to Mary or the saints, it explicitly says there's "one mediator between God and humanity, the man Christ Jesus," and they worry that even respectful requests to the dead risk blurring a line Scripture doesn't clearly permit crossing.
Both sides agree Mary deserves real honor as Jesus's mother. The disagreement is whether that honor should include actually addressing her in prayer.
Bible quotations in these answers are generally taken from the Christian Standard Bible®, Copyright © 2017 by Holman Bible Publishers. Used by permission. Christian Standard Bible® and CSB® are federally registered trademarks of Holman Bible Publishers. Some biblical wording may be paraphrased or follow other translations.