Interesting replies, everyone. Thanks for your input so far.
I would add, I do believe that eternal security has Old Testament roots, in the form of God's relation to Israel. When God wanted to destroy Israel at Sinai for their rebellion and start over with Moses, Moses interceded for them and asked God not to destroy them because of the covenants sake.
I believe OT souls were saved in the same way as we are today. In short, believing God.
"Abraham believed God, and it was counted unto him as righteousness".
Of course, they didn't fully understand the sacrificial atonement of the future Messiah. But the important thing is, they believed God, and righteousness was imputed to them.
This is the root of the gospel, I believe. 1 John 5:10 says "He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son."
I think it could be said of us Christians the exact thing which was said of Abraham. By believing in Christ, thus believing the "record that God gave of his Son", we are Believing God, and receiving imputed righteousness.