What does Jesus mean when he says that the Scriptures testify of him? What does he mean when he says that Moses wrote about him? Well, Irenaeus of Lyons (you can hear an introductory talk about him here) addresses this in ‘Against Heresies’ (chapter 10, fourth book), and as my last post was about Justin Martyr, I felt I just had to post something about Irenaeus:
Chapter X.—The Old Testament Scriptures, and those written by Moses in particular, do everywhere make mention of the Son of God, and foretell His advent and passion. From this fact it follows that they were inspired by one and the same God.
Wherefore also John does appropriately relate that the Lord said to the Jews: “Ye search the Scriptures, in which ye think ye have eternal life; these are they which testify of me. And ye are not willing to come unto Me, that ye may have life.” (John v. 39, 40). How therefore did the Scriptures testify of Him, unless they were from one and the same Father, instructing men beforehand as to the advent of His Son, and foretelling the salvation brought in by Him? “For if ye had believed Moses, ye would also have believed Me; for he wrote of Me;” (John v. 46) [saying this,] no doubt, because the Son of God is implanted everywhere throughout his writings: at one time, indeed, speaking with Abraham, when about to eat with him; at another time with Noah, giving to him the dimensions [of the ark]; at another; inquiring after Adam; at another, bringing down judgment upon the Sodomites; and again, when He becomes visible, ( See Gen. xviii. 13 and Gen. xxxi. 11, etc) and directs Jacob on his journey, and speaks with Moses from the bush.(Ex. iii. 4, etc.).
And it would be endless to recount [the occasions] upon which the Son of God is shown forth by Moses. Of the day of His passion, too, he was not ignorant; but foretold Him, after a figurative manner, by the name given to the passover; and at that very festival, which had been proclaimed such a long time previously by Moses, did our Lord suffer, thus fulfilling the passover. And he did not describe the day only, but the place also, and the time of day at which the sufferings ceased, and the sign of the setting of the sun, saying: “Thou mayest not sacrifice the passover within any other of thy cities which the Lord God gives thee; but in the place which the Lord thy God shall choose that His name be called on there, thou shalt sacrifice the passover at even, towards the setting of the sun.” (Deut.xvi.5, 6).