The Old Testament is the Gospel in bud

...let us beware of under-valuing the Old Testament. There has arisen of late years a most unhappy tendency to slight and despise any religious argument which is drawn from an Old Testament source, and to regard the man who uses it as a dark, benighted, and old-fashioned person. We shall do well to remember that the Old Testament is just as much inspired as the New, and that the religion of both Testaments is in the main, and at the root, one and the same. The Old Testament is the Gospel in the bud; the New Testament is the Gospel in full flower. The Old Testament is the Gospel in the blade: the New Testament is the Gospel in full ear. The Old Testament saints saw many things through a glass darkly: but they looked to the same Christ by faith and were led by the same Spirit as ourselves. Let us, therefore, never listen to those who sneer at Old Testament arguments. Much infidelity begins with an ignorant contempt of the Old Testament.

— J. C. Ryle, Sabbath: A Day to Keep