"preached the gospel unto Abraham" !?
Some people are under the impression that in the "old testament" men were saved by keeping the law. Some think that the ceremonial sacrifices actually atoned for their sin. Some just think that there was a different way of salvation before Christ.
But there never was a "before Christ". "Before Abraham was, I AM" (John 8:58). And there was never a time when GOD's plan of salvation was different. It was, and is, and always will be by faith alone through Christ alone. There is only one way of salvation; one mediator, Jesus Christ (1 Timothy 2:5)
Acts 4:10-12 KJV Be it known unto you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead, even by him doth this man stand here before you whole. 11 This is the stone which was set at nought of you builders, which is become the head of the corner. 12 Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.
Everyone who ever was saved, and ever will be saved, is saved by faith in Jesus Christ. The gospel to Adam and Eve, to Noah, to Abraham, and everyone else, is the gospel of the promise of the redeemer, Jesus Christ.