Were Old Testament believers saved even though they could not enter heaven?
Yes. Old Testament believers were saved for eternity, but they could not go to heaven before the cross. They went down to paradise, because the atoning blood of Christ alone removes the stain of sin. Animal blood could not take sin away, so they were not yet ready for God's presence.
Les teaches that Hades, or hell, is described by Abraham as a place with a great gulf fixed. On one side was torment, but on the other side was paradise. Before the cross, all people from Adam lived and died, and even believers, with two exceptions, did not go up to heaven. The exceptions were Enoch and Elijah, whom God took up, showing that God can make exceptions. Other than those two, all Old Testament believers could not go to heaven; they had to go down to paradise. This was because the atoning blood of Christ is the only thing that would remove the stain of sin. Animals' blood could not take it away. So these Old Testament believers were saved for eternity, but they were not ready for God's presence because their sins had not been atoned for by the blood of Christ. Therefore they went down to paradise. The lost from all ages, including today, still go down into hell, into the torment side. When Christ told the thief, "Today thou shalt be with me in paradise," He was speaking of that place in the centermost part of the earth, where He would be three days and three nights. Paul puts his stamp of approval on this, which is why Les is comfortable teaching it.