The whole gospel boils down to this truth. Because our sins have separated us from God, we need a “mediator” to bring us back to God. Because the “sin gap” is eternally wide, we need someone from heaven who is himself eternal to bridge the gap for us.
Jesus is the only one who could bridge that gap. By his death, he paid for our sins & bridged the gap that separates us from God. By his resurrection, he proved he is the Son of God. So we say & believe that no other mediator is necessary & no other mediator is possible.