1 All who believe and are assured that grace and truth were obtained through Jesus Christ, and who know Christ to be the truth, agreeably to His own declaration, “I am the truth,” derive the knowledge which incites men to a good and happy life from no other source than from the very words and teaching of Christ. And by the words of Christ we do not mean those only which He spoke when He became man and tabernacled in the flesh; for before that time, Christ, the Word of God, was in Moses and the prophets.
For without the Word of God, how could they have been able to prophesy of Christ? And were it not our purpose to confine the present treatise within the limits of all attainable brevity, it would not be difficult to show, in proof of this statement, out of the Holy Scriptures, how Moses or the prophets both spoke and performed all they did through being filled with the Spirit of Christ. And therefore I think it sufficient to quote this one testimony of Paul from the Epistle to the Hebrews, in which he says: “By faith Moses, when he had come to years, refused to be called the son of Pharaoh's daughter; choosing rather to suffer affliction with the people of God, than to enjoy the pleasures of sin for a season; esteeming the reproach of Christ greater riches than the treasures of the Egyptians.” Moreover, that after His ascension into heaven He spoke in His apostles, is shown by Paul in these words: “Or do you seek a proof of Christ who speaks in me?”
Source: De Principiis (New Advent)