Adam, Abraham and Moses were Christians, in my opinion, though it is obviously anachronistic to refer to them as such.
This, in my own little opinion, is the patristic witness. The Fathers talk about the revelation of the Trinity in the OT and seem to say that, although the fullness of it is unrealized until Christ, it wasn't something completely unknown to the saints of old, either.
It is definitely my opinion, as well, that Orthodox Christianity is
the religion of Abraham, Isaac and Jacob. It is not a schism from that faith, just the natural progression thereof. Rabbinic Judaism, as it is known today, is the schismatic faith. The first heretics and rejectors of Christ. That sounds harsh, but it's not anti-Semitic. The Jews were God's chosen people, and are still free and welcome to join the Church (Christ Himself, all of the 12 Apostles and many first-third century Christians were Jews!) but that they are no longer, as a people, the elect of God. That covenant is voided, and a new one established.