THE SHEMA: Hear, O Israel: The LORD our God is one LORD, is the great cry of Judaism. It is an affirmation and declaration of faith in one God, Deuteronomy chapter 6, verse 4.
Jesus makes the same declaration that was made in the old testament when questioned by a scribe on what the first commandment is, “And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord,” Mark 12:29.
And as much as there is only one God, there is only one Mediator: God Himself, in the person of Jesus the Christ.
Thomas, Peter and especially Mary, though blessed, are not Mediators. For the Bible says in the Book of First Timothy Chapter 2, and verse 5: For there is one God, and one mediator between God and men, the man Christ Jesus. And the Book of Hebrews Chapter 12, verse 24: And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel.
It was the sacrifice of Jesus on the cross which was an appropriate propitiation (appeasement) for the sins of all humanity. And it is only fair that the sacrifice-Jesus- gets the right to advocate and mediate, and not anybody else.