Read "Search for the Messiah" ISBN 9367285078. You'll be surprised. Very helpful book on this topic.
Orthodoxy is the same religion as Abarahm's. The Fathers didn't view Christianity as a new religion. When we go to heaven ,we will understand much more than we understand here. That doesn't mean we'll have converted to a new religion, it just means we'll have moved to the next phase of it. But we'll be in the same relationship with the same God, just knowing Him more fully. Likewise, when Christ was incarnate and completed revelation, He did not start a new religion, but revealed more of the truth to those who follow Him. The fact that some rejected the truth and kept on worshipping the old way doesn't mean they're the old religion... the old religion was preparing them for His coming, and if they rejected Him when He came, they apostated and left the religion of their fathers which was anticipating Him. Those who remained faithful are the true Israel.
You won't accept this concept though because you don't see the unity of the two testaments. In part this is because we are lazy and don't know the Bible, in part this is because the MT that the KJV we all read came from was watered down centuries after Christ came to decrease the references to Him.
If you take the time to read Search for the Messiah, you will be surprised. He does a great job of showing simply how the OT did point to Him, and how the people were expecting Him. Expectation of the coming of the Messiah was at an all-time high when Christ came, because they knew when he would come, from Daniel. The whole OT prepares the way for Him and speaks of Him, if we will study diligently and open our eyes, we'll see it.