But belief in Moses not only does not show our belief in the Spirit to be worthless, but, if we adopt our opponents' line of argument, it rather weakens our confession in the God of the universe. "The people," it is written, "believed the Lord and his servant Moses." Moses then is joined with God, not with the Spirit; and he was a type not of the Spirit, but of Christ. For at that time in the ministry of the law, he by means of himself typified "the Mediator between God and men." Moses, when mediating for the people in things pertaining to God, was not a minister of the Spirit; for the law was given, "ordained by angels in the hand of a mediator," namely Moses, in accordance with the summons of the people, "Speak thou with us, ...but let not God speak with us." Thus faith in Moses is referred to the Lord, the Mediator between God and men, who said, "Had ye believed Moses, ye would have believed me."
Source: ON THE SPIRIT 14.33