For there is one God, and there is one mediator between God and men, the man Christ Jesus, — 1 Timothy 2:5

That's the non-negotiable of the christian faith. Not many mediators, not a system of priests and saints—one mediator. And he's not just God, but the man Christ Jesus—fully able to represent us because he shared our flesh. The Lord is one, and a mediator implies more than one party. Moses stood in the gap and turned back God's wrath, but Jesus didn't just stand in the gap—he became the gap-filler, the perfect high priest.