I suppose that since you believe in the Bible, you are also a Christian. So I want to start off with, I totally agree with you that Jesus is the one mediator, the bridge between us and God the Father.
There are however many things that all Christians practise today that are not explicitly stated in the Bible. They are however found in the earliest records of the Church outside of the Bible. Some examples are the archaeological evidence of the earliest church buildings, paintings, liturgical books, other writings like the Didache (Teachings of the 12 Apostles dated back to end of 1st century).
One of such beliefs is the Trinity. The Bible mentions Father, Son and the Holy Spirit, yes, but nowhere does it talk about Trinity. However, as far as I know all Christians believe in the Trinity.
Another thing is congregational intercession. The Acts mentions the disciples prayed together. And when Peter was in prison, it specifically mentioned the Church was praying for him. However, nowhere does it say that one should ask for others to intercede.
For him. However, the earliest liturgical books indicate that the Church since the very beginning always gathered to pray for one another. And correct me if I’m wrong: all the Protestant brethren I’ve met also regularly pray for one another, and also ask for one another’s intercession.
And if we ask for the living on earth to intercede for us, why not also ask for the living in heaven to intercede for us? You see, the Church did not “create” the idea of asking the saints to intercede for us. It was a natural extension of the practice of intercession already existed among the living. If I have been going to this holy person for intercession while he is living among us, and I believe that after he is martyred he would go straight to heaven, then naturally I will continue to go to him for prayers after he suffered. And if you care to look up the annals of the Church, it was quickly discovered widely by the faithful that intercessions through saints brought about many miracles.
If intercession through the saints is wrong, then necessarily intercession through the living is also wrong. So when you gather with your brothers and sisters, please do not pray together for anybody.
Yet it is very clear that it was Jesus’ instructions and desire that we must gather as Church and as Church to pray and to minister to and with one another. Praying together with the Church we see here below is not wrong, and it does not take us away from Jesus. It is what Jesus wants and it in fact brings us closer to Jesus. Praying with the saints is the same. They are part of the family.