Prayer to the Saints and the Mediatorship of Christ
Q: I always understood the Bible to say that there is one mediator between God and Man, the man Jesus Christ. For this reason, it seems wrong to me to ask Mary to intercede for us.
A: Jesus is certainly the only Mediator in the sense that he his the only God-man, the only Person who serves as a bridge between the human and the divine in that way, as 1 Timothy 2:5 (the verse you quote) indicates.
However, Jesus’ unique role as the only God-man does not mean that nobody else gets to pray for us. This is clear if you read 1 Tim. 2:1-4, in which Paul says that we should all pray for everybody. So in the very same passage that Paul describes Jesus as the one Mediator, he also says that we should all pray and intercede for others. This is what Mary and the other heavenly saints do for us, just as people down here on earth do.
When you ask another Christian here on earth to pray for you, you are asking an earthly saint to intercede on your behalf. When you ask Mary to pray for you, you are doing the exact same thing, only it is a heavenly saint instead. Protestants ask earthly saints to pray for them; Catholics simply broaden this to all of the family of God and ask out heavenly brothers and sisters in Christ to pray for us as well as our brothers and sisters here on earth.
In fact, we know they pray for us already because, if you read Revelation 5:8, you will see the twenty-four elders (who represent the leaders of the people of God in heaven) offering to God the prayers of the saints. Thus we have the heavenly saints offering to God the prayers of the earthly saints. Thus we only ask our heavenly brothers and sisters to do what they are already anxious to do on our behalf, just as our earthly brother and sister Christians wish to pray for us whenever we are in need.