(yeah, yeah, another post not by me...soon)
It is a great mistake to say that Christianity, as over against the old dispensation, was a "new religion"; indeed, it is a mistake to say that Christianity is a religion at all, among other religions. On the contrary, there is just one revealed religion, and the revelation that is at the basis of it is recorded in both the Old and New Testament. The Old Testament saints were saved in just the same way as that in which the New Testament saints are saved--namely, by the death of Christ--and the mans by which the Spirit of God applied to them the benefits of Christ's death was exactly the same as the means by which the same Spirit applies those benefits to Christians today--namely, faith. The Old Testament saints, like Christians today, received the gospel of the grace of God; and, like the New Testament saints, they received it by faith. The only difference is that the gospel was proclaimed to the Old Testament saints by way of promise, while to us it is proclaimed by way of narrative of what has already been done. Immediately after the Fall of man, the plan of God for salvation began to be executed--with the promise contained in Gen 3:15--and the men who are saved in accordance with that plan are not adherents of "a religion" among other religions; they are not men who have built upon a common human fund of "religion" certain special religions known as "Judaism" and "Christianity," but they are men to whom God has supernaturally revealed and supernaturally applied His saving work. That one revealed "religion" does not differ form the religions of mankind merely in degree; its supremacy does not consist even in being the one perfect religion as over against the imperfect ones; but it is different from the religions of mankind because, while they represent man's efforts to find God, this "religion" is built upon the sovereign and gracious and entirely unique act by which God found man and saved him from the guilt and power of sin.--J. Gresham Machen