The Bible nowhere instructs believers in Christ to pray to anyone other than God. Nowhere in Scripture are we encouraged to ask individuals in heaven for their prayers. The Bible never mentions anyone in Heaven, other than Jesus, mediating on behalf of praying believers. First Timothy 2:5 declares that there is only mediator for us before God and that is Christ Himself. The Holy Spirit, who indwells believers, also intercedes for us, especially when we do not know what to pray (Romans 8:26-27). The Bible never indicates that Mary can hear our prayers. She is not omniscient as God is (Psalm 139:1-6). Even though she is a glorified being in Heaven, she remains finite with limitations. She is not equal with God and should not be put a position to be so (Exodus 20:3-4). We do not need to pray to anyone other than God because He is always ready to hear our prayers and welcomes our coming to Him (Philippians 4:6-7). We are told in Hebrews 4:16 that we can 'approach God's throne of grace with confidence.' God is all-powerful, all-loving, and all-knowing therefore we should only pray to Him.
There are two views among Catholics as to their interaction with Mary through prayer. Some Roman Catholics, though they deny doing it, actually pray to Mary. Others say they pray with her. Those who pray with her hold that a prayer addressed to Mary is just a way of asking someone in Heaven to pray for people on earth. In their eyes, this is no different from asking someone on earth to pray for someone.
The Catechism of the Roman Catholic Church, second edition, seems clear enough that prayers on earth can indeed be directed to Mary: “When we pray to [Mary], we are adhering with her to the plan of the Father, who sends his Son to save all men. Like the beloved disciple we welcome Jesus’ mother into our homes, for she has become the mother of all the living. We can pray with and to her” (part 4, § 1, ch. 2, art. 2, ¶ 2679, emphasis added). Two main prayers that Catholics pray to Mary are the Memorare and Hail, Holy Queen. Both of these prayers address Mary as one who can hear prayers, answer them in her own authority, and operate miraculously and powerfully from Heaven. To assign such authority to Mary makes her into an idol; God warns against idolatry in the Ten Commandments (Exodus 20:3-4). We are to worship Him alone. God welcomes our prayers only to Himself. We can approach Him with confidence (Hebrews 4:16), knowing that our all-powerful and loving Father is ready to hear us and give us the guidance and peace that only He can supply (Philippians 4:6-7).