The excellent Marc Lloyd has posted the juciest quotation on Christ the Mediator of all revelation. It's from Ronald Wallace's book Calvin's Doctrine of Word and Sacrament. Here he is summarizing Calvin's view especially of christocentric revelation in the OT.
The Mediator of all revelation between God and man in the Old Testament is the Word of God, the second person of the Trinity, the same Christ who became incarnate in Jesus of Nazareth. Throughout the whole national history of Israel, it was always He, the Son of God, who dealt with His people in judgement and mercy, bringing them, with His Presence in their midst, light and life and salvation. Calvin asserts positively that Christ, the Word of God, who "remains with God perpetually one and the same and who is God Himself" (Inst 1:13:7), was "always the bond of connection between God and man" (Comm on Gen 48:15), and "the source of all revelations" (Inst 1:13:7), being "always present in all the oracles" (Comm on Gen 16:10). He is equally emphatic in the frequent negative assertion, "Never did God reveal Himself outside of Christ" (Comm on Jn 5:23). "Nor indeed, had any of the saints ever had communication with God except through the promised Mediator." (Comm on Ex 3:2) "God formerly manifested Himself in no other way than though Him." (Comm on Gen 48:15) God never otherwise revealed Himself to the Fathers "but in His eternal Word and only begotten Son" (Comm on Is 6:1). The whole story of the Old Testament is thus the story of how Christ, the Word of God, breaks in upon the life of those whom He has chosen to make his people, and confronts them in these veiled forms through which they can come to know His nature and have communion with Him....
The frequent appearances of the "Angel of the Lord" as the representative of God to the Old Testemant Fathers, and as a guide of the people throughout their history is a sign that Christ is always fulfilling His Mediatorial office of saviour and revealer, and uniting even then the members of His Church to Himself as the Head through whom they are joined to God Himself. Calvin, following the "orthodox doctors" (Inst 1:13:10) on this point, identifies the "chief angel" who appears among the other angelic visitors to earth with "God's only begotten Son who was afterwards manifest in the flesh" (Comm on Ex 14:19). Even then He performed in a preliminary fashion "some services introductory to His execution of the office of Mediator" (Inst 1:13:10). "There is then no wonder," says Calvin, "that the Prophet should indictriminately call Him Angel and Jehovah, He being the Mediator of the Church and also God. He is God, being of the same essence with the Father; and Mediator, having already undertaken His Mediatorial office, though not then clothed in our flesh so as to become our brother; for the Church could not exist nor be united to God without a Head" (Comm on Zech 1:18-21). "The angel who appeared at first to Moses, and was always present with the people during their journeying, is frequently called Jehovah. Let is then regard it as a settled point that the angel was Son of God, and was even then the Guide of the Church of which He was the Head" (Comm on 1 Cor 10:9).
Calvin’s Doctrine of the Word and Sacrament (Edinburgh, Scottish Academic Press, 1995) first edition 1953, pp8-10