Abraham was saved at the same time the other old Testament saints were saved, at the time of the crucifixion. Keep in mind, Old Testament saints didn't go to "heaven" when they died. They went to "paradise" in the center of the Earth. It wasn't until Jesus Christ arose that they too were able to rise (some even stuck around for dinner with their families on their way up.) Everyone who ever lived or ever will live is saved by the blood of the Lord Jesus Christ.
At death, all pre-crucifixion (or possibly pre-resurrection) individuals went to the center of the earth at the time of their death (hades?) Some were in a place of comfort (paradise), some were in a place of torment (Hell). They were separated by a "great gulf." Of course, this was not a physical place, mind you, but a spiritual place with a physical location. Matthew 12:40 says "For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth." Jesus went there when he died and "redeemed" them with his blood.
The question becomes....what made the difference between someone who was comforted and someone who was tormented. I don't look at it from a standpoint of Salvation, at least not as we know it today. No one confessed belief in the coming Messiah and his blood atonement. Sure, it was alluded to in prophecy, but not even the disciples understood that Christ had to die for them.
Perhaps it was faithfulness to God. Perhaps it was a covenant he made with them as there God. Truthfully, I don't have an answer (nor do I need one.) I want to know, but God has not revealed it to us. The important thing to know for sure is that the blood of Christ saves all, past, present, and future.
